GB2137786A - Pipelined Digital Data Processor - Google Patents

Abstract

A pipelined digital signal processor includes a common data and control bus (101) and a source of instructions and data words. An arithmetic section (110) processes one data word with another data word through selected processing subsections performing operations according to an expression, thereby producing a resultant data word. Control circuits (e.g. IR-L, M, N) receive a single normal instruction during each processor cycle for controlling all processing subsections (112, 115, 116) operations. During each processor cycle, each processing subsection (112) performs an operation relating to a different expression than the other processing subsections (115 or 116) are performing during that processor cycle. All of the operations controlled by the single normal instruction are executed during a single processor cycle. The common bus (101) is time-shared during every processor cycle for transferring the single normal instruction from the source to the control circuits, for transferring data words from the source to the arithmetic section and for transferring the resultant data word from the arithmetic section to the destination. <IMAGE>

Description

1 GB 2 137 786 A 1

SPECIFICATION Pipelined Digital Signal Processor

The invention relates to a pipelined digital signal processor which may be more particularly described as a processor using a common data and control bus.

Digital computers typically include a memory, input output circuitry, a controller and an arithmetic section. The memory provides a source for a computer program to control the computer and for data to be operated on by the arithmetic section. The arithmetic section includes circuits which provide means for manipulating data in a predetermined manner. The controller provides control signals for regulating timing of operations and transfers of data to be operated upon. The input-output circuitry provides means for transferring information between the computer and external devices.

To increase computational speed, some digital computers are arranged for pipelined operation. In a pipelined operation the arithmetic section includes a collection of specialized circuits capable of working simultaneously but altogether forming a general purpose organization. These specialized circuits operate independently, each performing a specific task in a general purpose procedure. The pipelined operation divides a process into several subprocesses which are executed by the individual 15 specialized circuits. Successive ones of the subprocesses are carried out in an overlapped mode analogous to an industrial assembly line. New operands are applied at the input to the arithmetic section during each cycle. Different subsections of the arithmetic section perform their tasks in sequential order during subsequent cycles. A resultant is produced each cycle. Each specialized circuit performs its own task at the cyclic rate.

Control of a pipelined processor presents particularly perplexing problems because data and instructions become stacked up in pipelines during steady-state operation.

Heretofore a pipelined digital processor has been designed to transfer data words and instructions from memory to an arithmetic section and a control section in respective pipelined streams. Data words are transferred from memory by way of one bus to the arithmetic section.

Instructions are transferred from memory to the control section by way of another bus. These two separate busses alleviate bus contention and enable the pipelined data stream and the pipelined instruction stream to be transferred from section to section very rapidly. As a consequence, the rate of computation is enhanced.

A pr6b]em arises, however, when a processor designer desires to use this pipelined architecture 30 in a processor to be fabricated as a single integrated circuit chip. The logic circuitry and the bus structure requirements of the processor use so much space on- the chip that the chip becomes too expensive.

A solution to the problem is incorporated in an exemplary pipelined digital signal processor having a common data and control bus. The processor includes a source of instructions and data words. An arithmetic section processes one data word with another data word through selected processing subsections performing operations according to an expression, thereby producing a resultant data word. A destination receives the resultant data word from the arithmetic section. Control circuits receive a single instruction during each processor cycle for controlling all processing subsections operations. During each processor cycle, each processing subsection performs an 40 operation relating to a different expression than the other processing subsections are performing during that processor cycle. All of the operations controlled by the single instruction occur during a single processor cycle. The common bus is time-shared during every processor cycle for transferring the single instruction from the source to the control circuits, for transferring data words from the 45. source to the arithmetic section and for transferring the resultant data word from the arithmetic section 45 to the destination.

An embodiment of the invention will now be described by way of example, with reference to the drawing, wherein Figs. 1 and 2, when positioned as shown in Fig. 3, form a block diagram of a pipelined digital signal processor embodying the invention; Fig. 4 is a timing diagram; and Figs. 5 and 6 when positioned as shown in Fig. 7 form a processor function chart.

Referring now to Figs. 1 and 2, there is shown the overall architecture of a pipelined digital signal processor.

A read only memory 100 stores instruction and fixed data words. Instructions are transferred 55 from the read only memory by way of a common data and control bus 101 to instruction registers IR-C; ]R-L,M,N; and M-Sj. Parts of instructions are distributed to the instruction registers. Fixed data words, or coefficient words, are transferred from the read only memory by way of the common data and control bus 101 to a coefficient register 102. The register 102 is labelled REG X because the coefficients are identified hereinafter by the symbol x.

A random access memory 105 stores variable data words which may be stored therein either from an external source or from the output of the arithmetic section of this processor. The variable data words are transferred from the random access memory by way of the common data and control bus 101 to a variable data register 106. The register 106 is labelled REG Y because variable data words are 2 GB 2 137 786 A 2 identified hereinafter by the symbol y. By choice of the user, the random access memory may store coefficients used in place of fixed data words as well as the variable data words.

Registers 102 and 106, respectively, store a sequential stream of coefficient words and variable data words which are operands applied as inputs to an arithmetic section 110. These sequences of operands are processed in a pipeline fashion through a multiplier subsection 112, an accumulator subsection 115 and a rounding and overflow circuit subsection 116. A rounded output word is produced in a register 118 that is labelled REG W because rounded output words are identified by the symbol w hereinafter.

An output selector circuit 120 is included within the arithmetic section for choosing as an output word from the arithmetic section to the data bus 101 either the variable data word y stored in register 10 106 or the rounded output word w stored in register 118. The rounded output word w is a resultant of some process performed by the arithmetic section. The chosen output word can be transferred from either the register 106 or the register 118 by way of the common data and control bus 101 to a writeable destination, such as in the random access memory 105.

As previously mentioned, instructions for the digital signal processor are stored in read only memory 100. During each processor cycle, shown in Fig. 4, a single instruction automatically is read out of read only memory from a location having an address produced by an address arithmetic unit, or section, 124. The address from a program counter register PC in the address arithmetic section is applied by way of an address bus 128 to the address circuitry of the read only memory. Read only memory responds during each processor cycle by sending the single instruction thus fetched by way of the common data and control bus to the various control field, or instruction, registers IR-C, IR-1-,M,N, and IR- Sj associated with different sections of the processor.

Each instruction, or opcode, used in the digital signal processor includes a plurality of control fields, or control messages, each of which is given a designation such as 1, m, n, s and t to be used hereinafter. The control field register]R-L,M,N associated with arithmetic section 110 receives some of 25 the fields, such as instruction fields 1, m and n, respectively associated with control of multiplying, accumulating and rounding operations. The control field register IR-SJ, associated with the address arithmetic section 124, receives instruction fields s and t which relate to control of address register modification for controlling the fetching of operands x and y and the storing of the output word chosen by the selector circuit 120.

The address arithmetic section 124 includes two sets of registers 141 and 142, an address bus -1 7 latch 145, an adder 147 and an adder latch 150 interconnected by some busses.

One set of registers 141, including registers RX, RY, RD, and PC, is arranged to store memory addresses. An address stored in register RX can be used for accessing a coefficient word stored in a location in either random access memory or read only memory. An address stored in register RY can be 35 used only for accessing a variable data word stored in a location in random access memory. An address stored in the register RD can be used for writing a resultant data word into a destination, such as a location in random access memory. An address stored in the program counter register PC is used for accessing the next instruction or fixed data word from the read only memory.

The second set of registers 142 is arranged to store variable increment values to be used for 40 incrementing automatically addresses stored in registers RX, RY and RD. Alternatively, the stored addresses may be incremented by one of a set of fixed value increments.

Operations of the digital signal processor are controlled by two types of instructions. Normal instructions are used most of the time. They control the performance of arithmetic operations during signal processing. Another type of instruction, used occasionally, is called an auxiliary instruction. One 45 specific auxiliary instruction controls the loading of an address register or an address increment register in the address arithmetic section.

It is assumed that a start up sequence of instructions is stored in the read only memory starting at an initial address and that a reset circuit sets the program counter register PC to the initial address.

Following the reset operation, typically there is a sequence of instructions for storing additional addresses in the address registers RX, RY and RD and increment values in increment registers R], RJ and RK. These registers are set by auxiliary instructions. Ordinarily the values stored in the registers R], RJ and RK are retained therein throughout a program while the values in.the registers RX, RY and RD are modified from time to time during the execution of a sequence of normal instructions.

After the processor is reset and the address and increment values stored, the processor can run a 55 valid program for processing digital signals. Most of the instructions used for processing signals are normal arithmetic instructions.

Information in each of the registers RX, RY, RD, PC, R], RJ and RK can be set to any specific value by an auxiliary instruction. For example, a first instruction to load address register RY specifies that some processor register is to be loaded or set.

In this first instruction, a control field c contains the required information. This control field c is stored in an instruction register IR-C during the instruction fetch cycle.

A fixed data word, associated with the first instruction and loaded into the address arithmetic section 124 during the processor cycle in which that instruction' is decoded, provides information identifying which address register is to be loaded and fixing the increment value to be. loaded. The 65 A 3 GB 2 137 786 A 3 control field and value field are transferred from memory by way of the common data and control bus

101 to the control field register XSR and the value field register XSL.

While the first instruction is being executed, the control field in the register XSR is decoded in a decoder 157 to select the proper address register. From register XSL the value to be loaded into the address register RY is applied to the registers 141 and 142 through a selector circuit 158 and a bus 5 in the execution cycle of the first instruction.

A second instruction to load increment register R] specifies that a process register is to be loaded or set. As in the just described example of setting the address register RY, a fixed data word similarly associated with the second instruction provides a control field to identify the register to be set and a value field to establish the value to be loaded. The fields of the fixed data word are applied from the 10 register XSR through the decoder 157 and the. bus 137 to determine the increment register selected in the set of registers 142 and from the register XSL through selector 158 and bus 160 to establish the value to be loaded in the selected increment register during the execution cycle of the second instruction.

During the processing of both normal and auxiliary instructions, control fields s and t from the 15 instruction are stored in the instruction register M-SJ when that instruction is fetched. These fields are decoded in a decoder 152 during the next processor cycle with the decoded information being latched in an AAU control circuit 154. This decoded information is applied over a bus 135 to the sets of registers 141 and 142 during the instruction execute cycle, or second processor cycle, after the fetch.

Both an address register and an increment register or a fixed increment are selected by the information 20 on bus 135. The address is applied to the address bus latch 145 and to the input of an adder 147. The increment value simultaneously is applied to the other input of the adder 147, which increments the address and stores it for one machine state in an adder latch 150. During the following machine state, the incremented address is applied by way of a bus 136 to the set of address registers 141.

Simultaneously during the processing of a normal instruction, part of the information in the fields s and t is applied through a single machine state delay in a delay circuit 155. This delayed information provides selection information for determining which of the address registers 141 is to be written after the just described addressing operation. In the following machine state, the delayed information is decoded in a decoder 157 and applied over a bus 137 to the address registers 141. At this time, the incremented address stored in the adder latch 150 is written into the selected address register thus 30 post modifying the address.

During the processing of an auxiliary register set instruction, the above described operation for writing a post modified address back into an address register may be pre- empted by the register set operation. Pre-empting is accomplished by the decoder 157 in response to information applied thereto from logic circuit 122 by way of a path 138, AAU control circuit 154 and delay circuit 155.

When the register set instruction pre-empts the writing of an address register, the Information for selecting the address register is applied from register XSR through the decoder 157 and bus 137 to the address register set 141. Simultaneously from the register XSR, information is applied through decoder 157 and bus 137 for selecting information on bus 160 in lieu of information on bus 137.

The address arithmetic section 124 transmits addresses by way of the address bus latch 145 for 40 accessing locations in memories 100 and 105, generates new addresses in the adder 147 and sets the address registers RX, RY, RD and PC.

Referring now to Fig. 4, the diagram shows that addresses are transmitted to memory as a series of four addresses being transmitted during each processor cycle. One of the addresses is transmitted during each of four machinelstates during each processor cycle. The first address transmitted during 45 the first machine state is the address stored in the program counter register PC. As indicated in Fig. 4, this address is transmitted automatically during the first machine state of each processor cycle. The second address transmitted during the second machine state is the address stored in register RD or in register RX. The third address transmitted during the third machine state is the address stored in register RX or in the program counter register PC. The fourth address transmitted during the fourth 50 machine state is the address stored in register RY.

Each address transmitted by the address arithmetic section is latched in the address bus latch during the mentioned machine states of the processor cycle. Also during those machine states, the addresses are incremented in the address arithmetic unit adder 147 by an increment value that is read out of one of the increment registers RI, RJ and RK or in the case of the address from register PC, 55 the address is incremented by +1. These incrementing operations are accomplished during the same machine state that the address is latched.

Identification of the selected address and increment registers is accomplished by applying the appropriate control fields to the instruction register I R-SJ prior to the addressing operation so that the appropriate coding is applied to access circuitry for both the address and the increment registers during 60 the machine state that the address is to be transmitted. Both the address and the value of the increment are read out and are summed by the adder 147. The resulting incremented address is stored in the adder latch 150 while the address is being trdnsmitted from the address bus latch 145.

Coding for identifying whichever address register was selected is transferred through the delay circuit 155 to the decode register circuit 157. Delay and decoding are designed so that the 4 GB 2 137 786 A 4 incremented address stored in the adder latch 150 can be written into the address register from which the transmitted address was fetched. Thus the transmitted address is post- modified or post incremented during the processor cycle when it is transmitted to the memories 101 and 105.

Turning now to Fig. 2, the arithmetic section 110 is organized for pipelined operations.

Coefficients words x and variable data words y are operands received from the memories by way of the 5 common data and control bus 101 into coefficient word register 102 and the variable data word register 106. The rounded output words w also are operands for some operations and are stored in the register 118. A new operand is received into each of those registers during every processor cycle of a normal instruction.

The arithmetic section 110 includes three subsections which are independently controllable in 10 response to different control fields 1, m and n. During the fetch cycle of an instruction, the fields 1, m and n are stored in an instruction register IR-I-ffl,N. In the next processor cycle, those fields are decoded in a decoder circuit 113 and the result stored in register REG F. During the following processor cycle, this information is transferred to an AU control circuit 114 for supplying control signals to various subsections of the arithmetic section. This latter processor cycle is the execution cycle of the instruction. The control signals provide information relating to which choices are to be made from processing options available in each of the subsections. The multiplier subsection 112 typically generates a product of two operands during each processor cycle. In a typical multiplication, one operand is the coefficient word x and the second operand is either the variable data word y or the rounded output word w.

Coefficient word x is a 1 6-bit word. These sixteen bits are taken into the register 102 from the most significant bit lines of the common data and control bus. A selection circuit 162 scans the sixteen bits of the coefficient word, from the least significant bit to the most significant bit, four bits at a time during each of the four machine states in every processor cycle. Another selection circuit 163 concurrently selects either a 20-bit variable data word y or a 20-bit rounded output word w.

Multiplication based on Booth's algorithm is performed. Thus a Booth logic circuit 165 responds to the successive 4-bit nibbles to produce control signals for the generation of partial products.

The output from the Booth logic circuit 165 during every machine state is latched into a register 166. This output is applied to a circuit 168 which produces the partial products by data selection.

These partial products are accumulated by adding to prior sums and carries. An adder 170 sums 30 the partial products with the prior sum and carry information storing a resulting 36-bit intermediate operand, or product word p, in a product register P. Associated registers S and C respectively store the sum and carry information produced during each processor cycle.

Because the arithmetic section is arranged for pipelined operation, the product register P receives a new intermediate operand, or product word, p during every processor cycle of normal instructions. This product word p is applied by way of a bus 172 as an intermediate operand to the input of the accumulator subsection 115.

In the accumulator subsection, the product word p is added with a 40-bit resultant output word a that may be shifted by a circuit 174 prior to application as an input to an adder circuit 175. The adder circuit 175 produces sum and carry information which is stored in register 177. The sum and carry information is stored in register 177 during every processor cycle. Carries are resolved by carry-look ahead logic in adder 178. Output from adder 178 is applied to an input of a logic circuit 180 together with the resultant output word a to generate the next subsequent value of the 40-bit resultant output word a to be stored in register A. Such a resultant output word is produced and stored in register A during each processor cycle of a normal instruction.

A portion of the resultant output word a is applied as an input to the rounding and overflow circuit subsection 116 in 1 0-bit slices. These slices are clocked through a rounding circuit 182 and an overflow logic circuit 184 to the 20-bit rounded output register W in three consecutive machine states of each processor cycle. In the fourth machine state, the value in the register W may be corrected for overflow if the value in the register A is too large to be represented in the 20-bit register W. Then the 50 rounded output word can be transferred through the common data and control bus 101 to a destination, such as a location in the random access memory 105 where it is stored.

The three subsections (multiplier, accumulator and rounding) of the arithmetic section accomplish their basic operations in one processor cycle each. Outputs of the.subsections are stored in registers every processor cycle so that the next subsection in line has a stable input to commence the 55 next subsequent processor cycle.

Control of the arithmetic section 110 and of the address arithmetic section 124 is accomplished by a pipelined stream of instructions applied from the memory 100 through the common data and control bus 101. As previously stated with respect to Fig. 4, a single instruction is read out of memory during each processor cycle of operation. Such an instruction includes several instruction fields, or control messages 1, m, n, s and t. Fields 1, m and n are transferred through the common bus 10 1 to the register IR-Lffl,N for controlling the subsections of the arithmetic section 110. Fields s and t are transferred through the common bus 101 to the register IR-Sj for controlling selection and incrementation of addresses stored in the registers RX, RY, RD and PC.

- 1 7 -i GB 2 137 786 A 5 A fuller appreciation of the arrangement for and operation by pipelined control of processing may be achieved by the following discussion of a specific example of operation.

A complete normal assembly language instruction includes all of the information required to perform a desired arithmetic operation. Assembly language instructions for the digital signal processor are designed to represent the control for access to the memory and the control for operation of the 5 arithmetic subsection and of the address arithmetic subsection. The arithmetic subsection continuously performs multiplication and addition operations. The normal arithmetic section operations are characterized by the following general expressions:

x. fly) +fa(a)-->a {--,,wl x. f(w)+f,,(a)--->al-wl, 10 where processor.

x is a 16-bit wide coefficient word usually fetched from read only memory. The coefficient word x also could be fetched from random access memory or from an input/output circuit 200 and ordinarily has a value for all arithmetic operations.

y is a 20-bit wide data word normally fetched from random access memory. Such a data word 15 also could be fetched from the input/output circuit 200.

a represents the 40-bit wide contents of an accumulator register A. In the accumulator register A, the least significant thirty-six bits are used to accumulate the product of a 1 6-bit by 20-bit multiplication. The four most significant bits provide overflow protection for the accumulation operation.

w is a 20-bit wide rounded or truncated output of the accumulator. The least significant bit of the rounded output w corresponds with the bit that is fourteenth from the least significant bit of the contents a of the accumulator. This correspondence of bits is consistent with an assumption that the data word y and the rounded output w are integers and that the coefficient word x usually is restricted within the bounds -2:5x<2.

f describes a function of either the data word y or the rounded output w. Such function can be the actual value, the sign, or the absolute value of either one of the variables y or w.

f,, generally describes a function of the contents a of the accumulator, such as a, -a, 0, 2a, etc.

The variables x, y, w and p are contained in arithmetic section registers X, Y, W and P, respectively.

The aforementioned general expressions imply that three operations are to be performed by the (1) One of the products p=x. f(y) or p=x. f(w) is formed and is stored in the product register P located at the output of a multiplier.

(2) An accumulation of a resultant word a=p+fa(a) is accomplished in the accumulator.

(3) Then if required, the resultant word a of the accumulator is rounded and the rounded output word w is written into the rounded output register W.

Each of these three operations is completed during one processor cycle of the digital signal processor. Typically during the operations, the coefficient word x has a value and a multiplication forms the product p. Also typically during each cycle, all three types of operations are performed concurrently 40 by different subsections of the arithmetic section. For some instructions one or more of the three operations may not occur. The operation performed by one subsection during one processor cycle is a partial evaluation of a different general expression than the expressions concurrently being evaluated in the other subsections.

Assembly language instructions are converted to machine language instructions which are stored 45 in the memory for actually controlling the digital signal processor. Because the operations are dependent upon one another and because all of the operations occur concurrently within the processor, it is important to know at all times what is stored in Various registers and what operation is to be performed thereupon.

- To avoid confusion regarding which values of the product word p and which values of the contents a of the accumulator are involved in any processor operation, the following order of operations is recommended when writing assembly language expressions representing them.

P4----X - f(y) {w4--a} a4--p+f,,(a) or P--X - f(w) 55 Then as the reader proceeds from left to right, the proper values of the product word p and of the contents a of the accumulator are more apparent. The proper values are the results of the last preceding operation which determined those values. Thus the value of the contents a of the 6 GB 2 137 786 A accumulator to be rounded into the rounded output w or to be used in anyfunction f,,(a), is the contents a of the accumulator at the end of the last previous accumulation. Similarly, the value of a product word p to be used in a current accumulation has a value determined in the last previous multiplication operation.

Because of the reasons given in the foregoing discussion of the order of processor operations, it is important that the information contained in the assembly language instruction be presented to the processor in proper order. Information presented in the following order is acceptable to the processor.

(1) A choice of destination is made. The word to be written to the destination is chosen from either the rounded output word w or the data word y. The chosen word can be written into the random 10 access memory or into the input/output circuit. The specific destination of the selected word is given. (2) As required by the 'instruction, there is a choice of whether or not to move the resultant word a into the rounded output w. (3) Pne accumulation operation is selected from a group of operations having a general expression 15 a=p+f,(a). (4) Specify a multiplication operation producing the product p=x. f(y) by indicating the source XSRC of the coefficient word x, the nature of the function f, and the selection of the data word y, together with the source YSRC of the data word y. Alternatively specify a multiplication operation producing the product p=x. f(w) by indicating the source XSRC of the coefficient word x, the nature of the 20 function f, and the selection of the rounded output w rather than the data word y.

The following exceptions apply to the above-mentioned left-to-right rule. When the rounded output w is selected for the multiplication, the value of the rounded output w is the value determined by the last rounding of the resultant word a as performed in a preceding instruction. If data word V is to be written and a source for data word y specified, the first step in execution of the instruction moves the data from the specified source into the data register Y. Thereafter any writing of this new value for 25 data word y can occur.

The following Table 1 summarizes the normal assembly language instructions that a programmer would use for preparing an assembly language program. The syntax of a language called C is used as the assembly language which is described in a text entitled. The C Programming Language by B. W.

Kernighan et a], Prentice-Hall, Inc., 1978. Each complete instruction is formed by choosing four statements, one statement from each column of Table 1 starting with the lefthand column and working toward the right. In the two leftmost columns, the word NOTHING is listed as a valid choice. When the word NOTHING is selected as a part of a complete instruction, the corresponding space in the instruction is left blank. Every complete assembly language instruction is terminated by a semicolon.

TABLE 1 35

Normal Assembly Language Instructions NOTHING NOTHING a=p p=XSRCYSIRC DEST=y w=a a=p+a p=XSRCw DEST=YSIRIC a=p-a p=XSRCc DEST=w a=p+2a p=XSRCabs(QWIRC) 40 a=p+Ca p=XSRICabs(M a=p+a/2 p=XSRCcsgin(YSIRC) a=p+a/8 P=XSRCcsgn(w) a=p&a In Table 1 the symbol DEST means a destination statement and is to be replaced in the assembly 45 language instruction by one of the following statements.

DEST rd++i rd++j rd++k 50 obuf 7 GB 2 137 786 A 7 where e.g., rd++i means that rd, which is the address of the location in random access memory pointed to by the contents of register RD, is post incremented by the contents i of register RL Interpretations of different ones of the foregoing destination statements are explained in more detail subsequently.

Also in Table 1, the symbol XSRC means a statement for the source of data word x, and the 5 symbol YSRC means a statement for the source of the data word y. Each of those two symbols in Table

1 is to be replaced, in any assembly language instruction, by one of the statements in the following two columns:

XSRC YSRC rx 15 x(old x) VALUE (immediate x) rx++i rx++j rx++k rx-- ibufx (rom+rx++i) (rom+rx++D ry++i ry++j ry++k (rom+rx++k) 20 &LABEL In the foregoing columns headed XSRC, the symbol VALUE represents a number that appears as an argument of an instruction, i.e., the 1 6-bit word immediately following the opcode in the read only memory. Such argument is addressed by the address stored in the program counter register PC. In other fetches from memory, the coefficient word x is addressed by the contents rx of the register RX.25 The notation (rom+rx...) is used to indicate that the contents rx of the register RX point to read only memory rather than the random access memory. The symbol &LABEL represents that the value read from memory source x is an address associated with a label in the program. Other expressions presented in the foregoing two columns for the sources of the coefficient word x and the data word y 3Q are presented in more detail subsequently.

With respect to forming complete assembly language instructions from the information presented in Table 1, some caution is suggested. If the expression DEST=YSIRC is desired in an instruction including the expression YSIRC from the rightmost column, then the expression DEST=y must be used in place of the expression DEST=YSIRC. If the rounded output w is to be used in the rightmost column, the expression DEST=YSRC cannot be selected from the leftmost column. Additionally, NOTHING should be selected from the leftmost column when the assembly language instruction is a normal instruction in which the source of the coefficient word x is located in random access memory.

In the preparation of a program, a programmer will first write out a series of general mathematical expressions or operations desired to be performed. These may take, for example, the 40 form X f(W)+fa(a)--.>a{-+wl.

Such a general mathematical expression is translated by the programmer into an assembly language statement which takes the following form:

S n m 1, t rd++j=w w=a a=p+a p=rx++iry++k; 45 8 GB 2 137 786 A 8 where 1 means an instruction field for controlling the formation of a product.

m means the instructHon field for performing an accumulation.

n means an instruction field for controlling a transfer operation from register A to register W with the required rounding.

s represents an instruction field identifying a write destination. In this example the destination is a memory location specified by the address stored in register RD. That address is post incremented and the result stored in the register RD.

t means an instruction field to fetch information from an address stored in an address arithmetic unit register, post-increment that address and store it back into that same register. -. 10 as follows:

The next step performed by the programmer is to skew in time the assembly language statement

Time S n m 1, t i P=rx++iry++k; i+l a=p+a i+2 w=a i+3 rd++j=w The resulting skewed assembly language statement, which appears diagonally on the time line of the leftmost column, is stated togetherwith skewed assembly language statements representing other 15 general mathematical operations. When these skewed assembly language statements are stated together, the resulting pieces of different statement which appear in the same row, or during the same interval such as interval i, form an assembly language instruction. In the assembly language instruction, the different pieces of information in the same interval are separate fields of that assembly language instruction. Each of these fields controls a separate subsection of the processor for performing a step in 20 the process of evaluation, as described by a portion of one of the general mathematical expressions.

An assembler program, which runs on a general purpose computer, operates on each assembly language instruction by moving the source fields two processor cycles earlier in the program than the rest of the fields in that same.assembly language instruction. This moving of the source fields is done to every assembly language instruction in the program. The resulting time line for the foregoing assembler 25 statement, as skewed by the programmer and the assembler will appear as follows:

Time S t n m i-2 X=rx++i y=ry++k i_l i P=X y i+l a=p+a i+2 w=a i+3 rd++j=w Referring now to Figs. 5 and 6, there is shown a time line diagram indicating how data is processed in the digital signal processor. In general, the diagram presents the flow of data through various subsections of the processor during the evaluation of one general mathematical expression together with parts of other mathematical expressions.

Before attempting to describe the operations represented we will first define symbols used. throughout the time line diagram of Figs. 5 and 6.

j it X, 9 GB 2 137 786 A 9 Ii 11(t) li(I) li(M) is a machine language instruction fetched from read only memory during a processor cycle, or interval, i and decoded within the processor during a processor cycle, or interval, i+1. In general the instruction i, affects operation of sections of the processor during a processor cycle, or interval, i+2. As previously mentioned each instruction contains the fields, or control messages, 1, mn, s and t.

represents the field t in the machine language instruction 1, for controlling the fetching of operands x,+, and y,+,. These fetches take place during the interval i+3.

represents the field 1 in the machine language instruction Ii for controlling the 10 computation of a product,,or intermediate operand, Pl+2 during the interval i+2. The product Pl+2 is a function of the operands X,+1 and Y,+ 1.

represents the field m in the machine language instruction 1, for controlling the accumulation of output from, or desired resultant word, a, +2 during the interval i+2. The resultant word a,+2 is a function of the last prior resultant word a,+, and 15 a product P,+i previously computed.

li(s) 11(s' t) is a field n in the machine language instruction for controlling the transfer of a rounded output word W1+2 during interval i+2. Rounded Output Wi+2 is a function of the last prior rounded output w,+, and the resultant word a,+, of the accumulator.

is a field in the machine language instruction for controlling the storing of the rounded output word w,+, and the modification of register stored addresses U,+2 during the interval i+2. The modified addresses are a function of the prior address u,+, and field 1,(s). The updated memory state M,+2 is a function of the field 11(s), the prior memory state M,+, register stored addresses u,+, and the 25 rounded output word w,+,.

is a combination of fields s and t within the machine language instruction. The fields control the modification of register stored addresses Ui+2 during the interval i+2. The modified addresses Ui+2 are also a function of the address u,+,.

X, and y, are operands fetched from memory during the interval i, under control of the field 30 t of the instruction li-, fetched from memory during the interval i-3. Instruction 1,-, is decoded during interval i-2 and controls processing during interval i-1 wherein the addresses for operands xi and yi are produced. As previously mentioned these operands are accessed from memory during interval i. They are processed through the multiplier during the interval i+ 1 under control of the field 35 /of the instruction [,-,, which is fetched during the interval i,-,. This produces the intermediate operand or product pi+1.

Pi+i ai+2 W43 represents the product formed by the multiplier during the interval i+ 1. This product is an intermediate operand which is used as an input to the accumulator for its operation occurring during the interval i+2. Product p,+, is formed in register P under control of the field The multiplier and multiplicand are the operands x, and y,.

represents the contents of the accumulator during the interval i+2. This is the desired resultant word al+2 for the expression being evaluated. The word al+2 is an input for the rounding and output circuit subsection for the interval i+3. The 45 rounding operation occurs under the control of the field li+, (n).

represents rounded output word w which is available in the register W and which can be stored into writeable memory during the interval i+4 under the control of the field ',+2(S).

In the diagram of Figs. 5 and 6, there is shown all of the processing activities of various processor 50 subsections of the digital signal processor together with time in processor cycles. Each column in the chart represents a different processor cycle, or time interval, of the processor. Information in each column. is closely related to some machine language instruction. Each row represents activities of a different processor subsection performing its assigned functions during operation of the digital signal processor.

GB 2 137 786 A 10 Since each row of the chart represents a different activity, we shall define those activities. The first row below the processor cycle headings indicates storage activities, i.e., memory fetches and stores. The second row presents the time at which instructions are decoded within the digital signal processor. The third row shows the computing of the product p by the multiplier subsection of the processor. The fourth row presents the accumulating of the resultant word a by the accumulator subsection of the processor. Row five presents activities of the rounding and overflow subsection of the processor, which produces the rounded output word w. The sixth row discloses activities associated with modifying addresses used for fetching data for the arithmetic processes.

The processing of the aforementioned general arithmetic expression can be traced through the various sections and subsections of the digital signal processor by reference to Figs. 5 and 6.

A first step in the processing of a general arithmetic expression is the fetching of operands for a multiplication. As previously mentioned, information relating to this fetch operation is placed by the assembler program into an interval earlier than the information associated with control of the multiplication operation. As a result of this assembler program function, every machine language instruction includes a control field for a fetch operation that fetches information from memory for processing to be controlled by a subsequent machine language instruction.

As an example of processing an instruction, consider processing a general expression having information relating to fetch operations for its operands included within an instruction fetched during the interval i-3 of Fig. 5. This instruction 11-, is shown in an emphasized box and is labelled with a subscript identifying the instruction as the instruction fetched during interval i-3. Each instruction shown in the processor function chart is similarly designated in accordance with the interval during which the instruction is fetched from memory. Also each instruction, shown in Figs. 5 and 6 includes several fields of control information. Each of those fields 1, m, n, s and t are shown in parentheses associated with the instructions in the first row representing the fetching and storing operations. A separate field or separate fields of an instruction are shown in other rows of the chart, e.g., 11(1) in the row for computing products and li(s,t) in the row for modifying the addresses.

During the interval i-2, the just fetched instruction li-3 is decoded by the processor, as shown in the emphasized box in the second row representing the decoding of instructions.

A fetch operation for the operands x and y, identified by the instruction 11-3, begins during the interval i-l. The fetch operation begins using an address specified in the instruction field li-3(t). When 30 that address is used, it is modified and stored back in the address arithmetic section as a function of the instruction field],-3(s,t) and the prior state U1-2 of the registers in the address arithmetic section.

This modification of addresses is shown in the emphasized box under the interval i-1. Fetch of those operands x and y is concluded during interval i when the specific operands xl and yi, identified by the instruction li-3, are read out of memory and are transferred by way of the common data and control bus 35 respectively to registers REG X and REG Y. These fetch operations are shown in the emphasized box under the interval i. The operand x, typically is read out of read only memory, and operand yi typically is read out of random access memory.

The address pointers, or the addresses stored in registers RX and RY, which were updated in the prior interval i-1 are used for accessing the operands from memory during the interval 1.

a M The first arithmetic operation to be performed on the operands xl and yi occurs during interval i+1. At this time the multiplier subsection responds to the instruction field]i-l(i) for computing an intermediate operand, or product, pi,l, as shown in the emphasized box under interval i+1. Such product pi,l is shown as a function of the operands x, and y, and of the instruction field

Instruction 11-1, which includes the field li-10), is fetched from memory during the interval i-1, is 45 decoded during interval i and controls sections of the processor during interval i+1.

The next step in evaluating the general expression is processed in the accumulator during interval i+2. This is shown in Fig. 6 in the fourth row representing the accumulation of the resultant word a in an emphasized box under the column designated interval i+2. A resultant word ai+, is shown to be a function of the prior resultant word a,+, stored in the accumulator, the just described intermediate operand, or product, p,,, and of the instruction field 1,(m).

If specified by the programmer and after the result is accumulated during interval i+2, that result is rounded and is stored in the rounded output register W. This rounding operation is shown under the interval i+3 in an emphasized box in the fifth row representing rounding of the output. The specific rounding operation occurs during interval i+3 where the rounded output wi+ , is shown as a function of 55 the last prior rounded output w,+, of the rounded output register W, the just described resultant word ai+, of the accumulator, and of the instruction field 1,,,(n).

A final step in processing the general expression is a writing of the rounded output Wi+3 into memory during interval i+4. This is shown in the emphasized box in the first row of the chart under the interval i+4. Writing a new memory state M,+4 is a function of the memory state Mi+3 for interval j+3 of 60 the last prior address register state ui+,, of the last rounded output wi+ 3 just described, and of the instruction field li+,(s) which was fetched during interval i+2 and decoded during interval i+3.

Rounded output wi+, contained in the rounded output register at the end of interval i+3 is transferred by way of the common data and control bus either to the random access memory or to a buffer in the input/output circuitry during interval i+4.

it 11 GB 2 137 786 A 11 At the same time that the memory write operation occurs during interval i+ 4, the address arithmetic section registers are updated based on information in the instruction fetched during interval i+2. The information used is included in the fields 1,JS,t) of the instruction li+ 2 that is fetched during interval i+2 and is decoded during interval i+3.

During the interval i+2, it is noted that the instruction 1, which was fetched during interval 1 controls the multiplier subsection, the accumulator subsection and the rounding and overflow subsection of the arithmetic section. This results from the instruction 1, being fetched in interval i, decoded in interval W and used for control during interval i+ 2. No parts remain for controlling subsections of the arithmetic section during subsequent intervals, as in prior pipelined control arrangements. Most of the column representing interval i+2 is emphasized with heavy lines so that 10 the reader readily can find several fields of the instruction 1, for controlling subsections of the arithmetic section during interval i+2.

Operands for the multiplier operation were fetched during the interval W which follows interval 1. The resulting product pi+, is formed during the next interval i+2.

A resultant word a,+, which is formed during that same interval 1+2 is a function of an earlier 15 resultant word. ai+, and an earlier product p,+,. This resultant word ai+, is a resultant word evaluated for a different general expression than the general expression being evaluated by forming the product Pi+2.

This concept can perhaps be better understood by the realization that the emphasized boxes forming a diagonal from the top of the column designated processor cycle!down to the fifth row in the column designated processor cycle i+3 relate to the evaluation of one general expression. A similar diagonal, 20 shifted one interval to the right in each column, relates to the evaluation of another different general expression.

Typically in a signal processing program. instructions are executed, in sequence, up to a point where the program counter PC, is set to the address value in the program store which is the location of the instruction of the beginning of the sequence. Thus the program will operate continuously in a loop 25 executing the same sequence of instructions repeatedly. Furthermore, fixed data words will be stored at memory locations where addresses are interleaved with location of instructions in the program sequence. In this way, as shown in Fig. 4, the address in the program counter register PC is used to address a fixed data word during state 2 of processor cycle i+1. The program counter then is incremented by the fixed increment +1 or is used to address an instruction, ',12, in state 0 of processor 30 cycle i+2. Again the program counter is incremented by the fixed increment, +1, and used to address the next fixed data word in state 2 of processor cycle i+2. Continuing, the program counter is incremented by +1 and is used to address instruction, 1,+, in state 0 of the processor cycle i+3 and so on until the end of the instruction sequence. At that time the program counter is set, by an auxiliary register set instruction, to the address of the first instruction in the sequence.

Claims (1)

1. CLAIM

   1. A pipelined digital signal processor responsive to source means providing a stream of instruction words for controlling routine processing operations and providing a stream of fixed data words and variable data words; comprising an arithmetic section for processing one of the fixed data words with one of the variable data words through selected operations occurring in sequential processor cycles and representing an expressi on producing a resultant data word; means for transferring the resultant data word from the arithmetic unit to a destination; control means (IR-L, M,N and IR-SJ) for receiving one instruction word during a first processor cycle (i=l) for determining which ones of the selectable group of operations occurs during a second processor cycle (i+2=3); a source means accessing arrangement including a single time-shared bus for interconnecting the source means 45 with the control means for transferring control words from the source means to the control means and for interconnecting the source means with the input of the arithmetic section for transferring fixed and variable data words from the source means to the arithmetic section, and a destination means accessing arrangement including the single time-shared bus for interconnecting the output of the -50 arithmetic section with the destination means for transferring resultant data words from the arithmetic 50 section to the destination means.

   Printed in the United Kingdom for Her Majesty's Stationery Office, Demand No. 8818935, 10/1984. Contractor's Code No. 6378. Published by the Patent Office, 25 Southampton Buildings, London, WC2A lAY, from which copies may be obtained.