URISC Computer Architecture and Design Spring 2008 Sreekumar Menon Pg1 Outline Introduction to URISC

Motivation Implementation Drawbacks

Work Pg2 on URISC Introduction URISC One stands for Ultra-RISC

instruction only !!!!!!!! No Opcode No need for it.. Interesting ??? Gets better, can be implemented with minimal hardware too

Introduction Instruction should execute Subtract Branch if less than equal Memory Operations needed Theorem states that any instruction that has

this capability can be used as an URISC instruction Computer Reference: The Ultimate Reduced Instruction Set Int.J.Elect.Enging Educ., Vol 25, pp 327-334 Pg 4 Motivation How much can RISC be reduced to ?

Main idea was to a create a fast, simple computer- THE HOLY GRAIL Simple Instructions means a simple hardware and a faster clock Eliminates the decode stages in other computers URISC is extreme in simplicity

URISC is Turing Compatible Being equivalent to a universal Turing machine essentially means being able to perform any computational task that takes finite input and returns finite output in finitely-many steps. ["Turing completeness." Wikipedia, The Free Encyclopedia.]

By creating other instructions based on subtract, branch if negative or equal it can be shown that URISC is touring compatible. Pg 6 URISC Instruction

b b a ( a,b are Registers) If b <=0 then PC = Branch Target Address else PC= PC + 1

Branch Addressing can be made relative or implicit 1st Operand = a Pg 7 2nd Operand = b Branch Target Address Machine Level

Interpretation : , ,

L: Instruction Label P: Jump Target Label A,B: Refernces to the memory where the operands are located Pg 8

Example Programs SUB(A,B) // B B - A, no branch SUB:A,B,END END: SETZERO(A) // A 0 SETZERO:A,A,END END: ...

ADD(A,B) // B A + B, Assumes T0 is set to 0 ADD0:A,T0,ADD1 ADD1:T0,B,END END: ... Pg 9 Example Programs MOVE(A,B) // B A Assumes T0 is set 0 MOV0: B,B, MOV1

MOV1: A,T0, MOV2 MOV2: T0,B, END END: ... JUMP(Target) // Unconditional Jump JUMP:T0,T0,Target Pg 10 Example Programs

BEQ(A,Target) //Assumes T0 == 0 // IF A == 0 Then PC = Target BEQ0:A,T0,BEQ2 BEQ1:T0,T0, END BEQ2:T0,T0, BEQ3 BEQ3:T0,A, Target END: ...

Pg 11 URISC is a Multicycle Processor Takes 4 clock cycles to complete a single instruction. During each clock cycle a different set of control signals is output from the control unit. These control signals effect the flow

of data in the processor/datapath. A counter counts what microinstruction we are on. For each value of the counter a different set of microinstructions is output. At the end of the instruction (after microinstruction 4 or earlier) the counter is reset to 0, and execution of the next instruction begins Pg 12 Hardware Implementation

URISC hardware can be implemented in various forms best suited for optimizing one instruction Pg 13 Hardware Implementation Hardware Implementation The

URISC Computer uses minimal hardware. To implement the instruction we need to: check to see if PC = 0 load the first operand increment PC load the second operand subtract the operand store result in to the second operand increment PC load target

increment PC if result is negative, set PC to target Pg 15 Cycle 1 PC check to 0 Cycle 2 Loading New Operand Cycle 3 PC Increment

Cycle 4 Load Second Operand Cycle 5 Substract and Store Cycle 6 Load Target Cycle 7 Load target IF PC ive Problems with the Holy

Grail URISC architecture is not competitive The cycles taken by URISC or the execution time per instruction is on an average 75% more than a MIPS multi-cycle architecture But that doesnt prove it to be suboptimal Pg Optimal Architectures

Class of optimal architectures can be thought of as a surface in a multidimensional computer design space Taking typical axes of the space to be processor complexity the program size for some benchmark, and the memory traffic required to execute that benchmark, its clear that URISC fares worse than any other architecture

Pg 24 Optimal Architectures The minimal ultimate RISC can only be proven to be suboptimal if a processor can be found that is better when measured along at least one axis of the design space while being no worse along any other axes. Pg 25

Work on URISC Steve Loughran formally defined, designed and built a 32-bit variant of this architecture as his final-year project at Edinburgh University in 1989 Adam Donlin has proposed using an Ultimate RISC as a host for a dynamically reconfigurable FPGA coprocessor in "Self Modifying

Circuitry -- A Platform for Trackable Virtual Circuitry" in Proceedings of FPL the 9th International Workshop, FPL99, Springer-Verlag, ISSN 03029743, Aug 1999. Work on URISC Paul Frenger wrote published a paper in ACM Sigplan Notices 35, 2 (Feb 2000) entitled "The Ultimate RISC: A Zero-Instruction Computer"; ACM Computer Architecture News, 16, 3 (June 1988), pages 48-55.

Univ. of Waterloo URISC: F. Mavaddat and B. Parhami, URISC: The Ultimate Reduced Instruct ion Set Computer Thank You Questions ???????????

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