Instructor: Erol Sahin

Y86 Instruction Set Architecture – SEQ processorCENG331: Introduction to Computer Systems8thLecture Instructor: ErolSahin • Acknowledgement: Most of the slides are adapted from the ones prepared • by R.E. Bryant, D.R. O’Hallaron of Carnegie-Mellon Univ.

Application Program Compiler OS ISA CPU Design Circuit Design Chip Layout Instruction Set Architecture • Assembly Language View • Processor state • Registers, memory, … • Instructions • addl, movl, leal, … • How instructions are encoded as bytes • Layer of Abstraction • Above: how to program machine • Processor executes instructions in a sequence • Below: what needs to be built • Use variety of tricks to make it run fast • E.g., execute multiple instructions simultaneously

OF ZF SF Y86 Processor State Program registers Condition codes Memory • Program Registers • Same 8 as with IA32. Each 32 bits • Condition Codes • Single-bit flags set by arithmetic or logical instructions • OF: Overflow ZF: Zero SF:Negative • Program Counter • Indicates address of instruction • Memory • Byte-addressable storage array • Words stored in little-endian byte order %eax %esi %ecx %edi PC %edx %esp %ebx %ebp

Y86 Instructions • Format • 1--6 bytes of information read from memory • Can determine instruction length from first byte • Not as many instruction types, and simpler encoding than with IA32 • Each accesses and modifies some part(s) of the program state

%eax 0 %esi 6 %ecx 1 %edi 7 %edx 2 %esp 4 %ebx 3 %ebp 5 Encoding Registers • Each register has 4-bit ID • Same encoding as in IA32 • Register ID 8 indicates “no register” • Will use this in our hardware design in multiple places

Generic Form Encoded Representation %eax 0 %esi 6 %ecx 1 %edi 7 addl rA, rB %edx 2 %esp 4 %ebx 3 %ebp 5 6 0 rA rB Instruction Example Addition Instruction • Add value in register rA to that in register rB • Store result in register rB • Note that Y86 only allows addition to be applied to register data • Set condition codes based on result • e.g., addl %eax,%esiEncoding:60 06 • Two-byte encoding • First indicates instruction type • Second gives source and destination registers

Instruction Code Function Code addl rA, rB xorl rA, rB andl rA, rB subl rA, rB 6 6 6 6 2 3 1 0 rA rA rA rA rB rB rB rB Arithmetic and Logical Operations • Refer to generically as “OPl” • Encodings differ only by “function code” • Low-order 4 bytes in first instruction word • Set condition codes as side effect Add Subtract (rA from rB) And Exclusive-Or

rA 8 rA rB rB rB rrmovl rA, rB 4 3 2 5 0 0 0 0 rA rB irmovl V, rB mrmovl D(rB), rA rmmovl rA, D(rB) V D D Move Operations • Like the IA32 movlinstruction • Simpler format for memory addresses • Give different names to keep them distinct Register --> Register Immediate --> Register Register --> Memory Memory --> Register

%eax 0 %esi 6 %ecx 1 %edi 7 %edx 2 %esp 4 %ebx 3 %ebp 5 Move Instruction Examples IA32 Y86 Encoding movl $0xabcd, %edx irmovl $0xabcd, %edx 30 82 cd ab 00 00 movl %esp, %ebx rrmovl %esp, %ebx 20 43 movl -12(%ebp),%ecx mrmovl -12(%ebp),%ecx 50 15 f4 ff ff ff movl %esi,0x41c(%esp) rmmovl %esi,0x41c(%esp) 40 64 1c 04 00 00 movl $0xabcd, (%eax) — movl %eax, 12(%eax,%edx) — movl (%ebp,%eax,4),%ecx —

Jump When Equal Jump When Not Equal Jump Unconditionally Jump When Greater Jump When Greater or Equal Jump When Less or Equal Jump When Less jge Dest jl Dest je Dest jmp Dest jg Dest jne Dest jle Dest Dest Dest Dest Dest Dest Dest Dest 7 7 7 7 7 7 7 0 4 3 2 1 5 6 Jump Instructions • Refer to generically as “jXX” • Encodings differ only by “function code” • Based on values of condition codes • Same as IA32 counterparts • Encode full destination address • Unlike PC-relative addressing seen in IA32

Y86 Program Stack Stack “Bottom” • Region of memory holding program data • Used in Y86 (and IA32) for supporting procedure calls • Stack top indicated by %esp • Address of top stack element • Stack grows toward lower addresses • Top element is at highest address in the stack • When pushing, must first decrement stack pointer • When popping, increment stack pointer • • • Increasing Addresses %esp Stack “Top”

pushl rA popl rA a rA b rA 0 8 0 8 Stack Operations • Decrement %espby 4 • Store word from rA to memory at %esp • Like IA32 • Read word from memory at %esp • Save in rA • Increment %espby 4 • Like IA32

call Dest Dest ret 8 9 0 0 Subroutine Call and Return • Push address of next instruction onto stack • Start executing instructions at Dest • Like IA32 • Pop value from stack • Use as address for next instruction • Like IA32

nop halt 0 1 0 0 Miscellaneous Instructions • Don’t do anything • Stop executing instructions • IA32 has comparable instruction, but can’t execute it in user mode • We will use it to stop the simulator

Y86 Code Generation Example First Try • Write typical array code • Compile with gcc -O2 -S • Problem • Hard to do array indexing on Y86 • Since don’t have scaled addressing modes /* Find number of elements in null-terminated list */ int len1(int a[]) { int len; for (len = 0; a[len]; len++) ; return len; } L18: incl %eax cmpl $0,(%edx,%eax,4) jne L18

Y86 Code Generation Example #2 Second Try • Write with pointer code • Compile with gcc -O2 -S • Result • Don’t need to do indexed addressing /* Find number of elements in null-terminated list */ int len2(int a[]) { int len = 0; while (*a++) len++; return len; } L24: movl (%edx),%eax incl %ecx L26: addl $4,%edx testl %eax,%eax jne L24

Y86 Code Generation Example #3 IA32 Code • Setup Y86 Code • Setup len2: pushl %ebp # Save %ebp xorl %ecx,%ecx # len = 0 rrmovl %esp,%ebp # Set frame mrmovl 8(%ebp),%edx # Get a mrmovl (%edx),%eax # Get *a jmp L26 # Goto entry len2: pushl %ebp xorl %ecx,%ecx movl %esp,%ebp movl 8(%ebp),%edx movl (%edx),%eax jmp L26

Y86 Code Generation Example #4 IA32 Code • Loop + Finish Y86 Code • Loop + Finish L24: movl (%edx),%eax incl %ecx L26: addl $4,%edx testl %eax,%eax jne L24 movl %ebp,%esp movl %ecx,%eax popl %ebp ret L24: mrmovl (%edx),%eax # Get *a irmovl $1,%esi addl %esi,%ecx # len++ L26: # Entry: irmovl $4,%esi addl %esi,%edx # a++ andl %eax,%eax # *a == 0? jne L24 # No--Loop rrmovl %ebp,%esp # Pop rrmovl %ecx,%eax # Rtn len popl %ebp ret

Y86 Program Structure irmovl Stack,%esp # Set up stack rrmovl %esp,%ebp # Set up frame irmovl List,%edx pushl %edx # Push argument call len2 # Call Function halt # Halt .align 4 List: # List of elements .long 5043 .long 6125 .long 7395 .long 0 # Function len2: . . . # Allocate space for stack .pos 0x100 Stack: • Program starts at address 0 • Must set up stack • Make sure don’t overwrite code! • Must initialize data • Can use symbolic names

Simulating Y86 Program • Instruction set simulator • Computes effect of each instruction on processor state • Prints changes in state from original • unix> yis eg.yo • Stopped in 41 steps at PC = 0x16. Exception 'HLT', CC Z=1 S=0 O=0 • Changes to registers: • %eax: 0x00000000 0x00000003 • %ecx: 0x00000000 0x00000003 • %edx: 0x00000000 0x00000028 • %esp: 0x00000000 0x000000fc • %ebp: 0x00000000 0x00000100 • %esi: 0x00000000 0x00000004 • Changes to memory: • 0x00f4: 0x00000000 0x00000100 • 0x00f8: 0x00000000 0x00000015 • 0x00fc: 0x00000000 0x00000018

CISC versus RISC CENG331: Introduction to Computer Systems8thLecture Instructor: ErolSahin • Acknowledgement: Most of the slides are adapted from the ones prepared • by R.E. Bryant, D.R. O’Hallaron of Carnegie-Mellon Univ.

CISC Instruction Sets • Complex Instruction Set Computer • Dominant style through mid-80’s • Stack-oriented instruction set • Use stack to pass arguments, save program counter (IA32 but not int x86-64) • Explicit push and pop instructions • Arithmetic instructions can access memory • addl %eax, 12(%ebx,%ecx,4) • requires memory read and write • Complex address calculation • Condition codes • Set as side effect of arithmetic and logical instructions • Philosophy • Add instructions to perform “typical” programming tasks

RISC Instruction Sets • Reduced Instruction Set Computer • Internal project at IBM, later popularized by Hennessy (Stanford) and Patterson (Berkeley) Fewer, simpler instructions • Might take more to get given task done • Can execute them with small and fast hardware Register-oriented instruction set • Many more (typically 32) registers • Use for arguments, return pointer, temporaries Only load and store instructions can access memory • Similar to Y86 mrmovland rmmovl No Condition codes • Test instructions return 0/1 in register

MIPS Registers

Op Ra Rb Rd 00000 Fn R-R Op Op Op Ra Ra Ra Rb Rb Rb Offset Immediate Offset R-I MIPS Instruction Examples addu $3,$2,$1 # Register add: $3 = $2+$1 addu $3,$2, 3145 # Immediate add: $3 = $2+3145 sll $3,$2,2 # Shift left: $3 = $2 << 2 Branch beq $3,$2,dest # Branch when $3 = $2 Load/Store lw $3,16($2) # Load Word: $3 = M[$2+16] sw $3,16($2) # Store Word: M[$2+16] = $3

CISC vs. RISC • Original Debate • Strong opinions! • CISC proponents---easy for compiler, fewer code bytes • RISC proponents---better for optimizing compilers, can make run fast with simple chip design • Current Status • For desktop processors, choice of ISA not a technical issue • With enough hardware, can make anything run fast • Code compatibility more important • For embedded processors, RISC makes sense • Smaller, cheaper, less power

Summary • Y86 Instruction Set Architecture • Similar state and instructions as IA32 • Simpler encodings • Somewhere between CISC and RISC • How Important is ISA Design? • Less now than before • With enough hardware, can make almost anything go fast • AMD/Intel moved away from IA32 • Does not allow enough parallel execution • x86-64 • 64-bit word sizes (overcome address space limitations) • Radically different style of instruction set with explicit parallelism • Requires sophisticated compilers

Logic Design and HCLCENG331: Introduction to Computer Systems8thLecture Instructor: ErolSahin • Acknowledgement: Most of the slides are adapted from the ones prepared • by R.E. Bryant, D.R. O’Hallaron of Carnegie-Mellon Univ.

a && b Computing with Logic Gates • Outputs are Boolean functions of inputs • Respond continuously to changes in inputs • With some, small delay Falling Delay Rising Delay b Voltage a Time

Acyclic Network Primary Inputs Primary Outputs Combinational Circuits Acyclic Network of Logic Gates • Continously responds to changes on primary inputs • Primary outputs become (after some delay) Boolean functions of primary inputs

Bit equal a eq b Bit Equality • Generate 1 if a and b are equal • Hardware Control Language (HCL) • Very simple hardware description language • Boolean operations have syntax similar to C logical operations • We’ll use it to describe control logic for processors HCL Expression bool eq = (a&&b)||(!a&&!b)

b31 Bit equal eq31 a31 b30 Bit equal eq30 a30 Eq B = Eq b1 Bit equal eq1 A a1 b0 Bit equal eq0 a0 Word Equality Word-Level Representation • 32-bit word size • HCL representation • Equality operation • Generates Boolean value HCL Representation bool Eq = (A == B)

Bit-Level Multiplexor • Control signal s • Data signals a and b • Output a when s=1, b when s=0 s Bit MUX HCL Expression bool out = (s&&a)||(!s&&b) b out a

s b31 out31 a31 s b30 out30 MUX B Out a30 A b0 out0 a0 Word Multiplexor Word-Level Representation • Select input word A or B depending on control signal s • HCL representation • Case expression • Series of test : value pairs • Output value for first successful test HCL Representation int Out = [ s : A; 1 : B; ];

MIN3 C Min3 B A s1 s0 MUX4 D0 D1 Out4 D2 D3 HCL Word-Level Examples Minimum of 3 Words • Find minimum of three input words • HCL case expression • Final case guarantees match int Min3 = [ A < B && A < C : A; B < A && B < C : B; 1 : C; ]; 4-Way Multiplexor • Select one of 4 inputs based on two control bits • HCL case expression • Simplify tests by assuming sequential matching int Out4 = [ !s1&&!s0: D0; !s1 : D1; !s0 : D2; 1 : D3; ];

2 1 0 3 Y Y Y Y A A A A X ^ Y X & Y X + Y X - Y X X X X B B B B OF OF OF OF ZF ZF ZF ZF CF CF CF CF A L U A L U A L U A L U Arithmetic Logic Unit • Combinational logic • Continuously responding to inputs • Control signal selects function computed • Corresponding to 4 arithmetic/logical operations in Y86 • Also computes values for condition codes

i7 D o7 Q+ C i6 D o6 Q+ C i5 D o5 Q+ C i4 D o4 Q+ I O C i3 D o3 Q+ C i2 D o2 Q+ C i1 Clock D o1 Q+ C i0 D o0 Q+ C Clock Registers Structure • Stores word of data • Different from program registers seen in assembly code • Collection of edge-triggered latches • Loads input on rising edge of clock

Rising clock State = y y Output = y   Register Operation • Stores data bits • For most of time acts as barrier between input and output • As clock rises, loads input State = x x Input = y Output = x

Comb. Logic 0 MUX 0 Out In 1 Load Clock Clock Load A L U x0 x1 x2 x3 x4 x5 In x0 x0+x1 x0+x1+x2 x3 x3+x4 x3+x4+x5 Out State Machine Example • Accumulator circuit • Load or accumulate on each cycle

valA Register file srcA A valW Read ports W dstW Write port valB srcB B Clock Random-Access Memory • Stores multiple words of memory • Address input specifies which word to read or write • Register file • Holds values of program registers • %eax, %esp, etc. • Register identifier serves as address • ID 8 implies no read or write performed • Multiple Ports • Can read and/or write multiple words in one cycle • Each has separate address and data input/output

Register file Register file y 2 valW valW W W dstW dstW x valA Register file srcA A 2 Clock Clock valB srcB B x 2 Rising clock y   2 Register File Timing Reading • Like combinational logic • Output data generated based on input address • After some delay Writing • Like register • Update only as clock rises x 2

Hardware Control Language • Very simple hardware description language • Can only express limited aspects of hardware operation • Parts we want to explore and modify • Data Types • bool: Boolean • a, b, c, … • int: words • A, B, C, … • Does not specify word size---bytes, 32-bit words, … • Statements • bool a = bool-expr; • int A = int-expr;

HCL Operations • Classify by type of value returned • Boolean Expressions • Logic Operations • a && b, a || b, !a • Word Comparisons • A == B, A != B, A < B, A <= B, A >= B, A > B • Set Membership • A in { B, C, D } • Same as A == B || A == C || A == D • Word Expressions • Case expressions • [ a : A; b : B; c : C ] • Evaluate test expressions a, b, c, … in sequence • Return word expression A, B, C, … for first successful test

Summary Computation • Performed by combinational logic • Computes Boolean functions • Continuously reacts to input changes Storage • Registers • Hold single words • Loaded as clock rises • Random-access memories • Hold multiple words • Possible multiple read or write ports • Read word when address input changes • Write word as clock rises

SEQuential processor implementationCENG331: Introduction to Computer Systems8thLecture Instructor: ErolSahin • Acknowledgement: Most of the slides are adapted from the ones prepared • by R.E. Bryant, D.R. O’Hallaron of Carnegie-Mellon Univ.

Byte 0 1 2 3 4 5 nop 0 0 addl 6 0 halt 1 0 subl 6 1 rrmovl rA, rB 2 0 rA rB andl 6 2 irmovl V, rB 3 0 8 rB V xorl 6 3 rmmovl rA, D(rB) 4 0 rA rB D jmp 7 0 mrmovl D(rB), rA 5 0 rA rB D jle 7 1 OPl rA, rB 6 fn rA rB jl 7 2 jXX Dest 7 fn Dest je 7 3 call Dest 8 0 Dest jne 7 4 ret 9 0 jge 7 5 pushl rA A 0 rA 8 jg 7 6 popl rA B 0 rA 8 Y86 Instruction Set

newPC SEQ Hardware Structure PC valE , valM Write back valM State • Program counter register (PC) • Condition code register (CC) • Register File • Memories • Access same memory space • Data: for reading/writing program data • Instruction: for reading instructions Instruction Flow • Read instruction at address specified by PC • Process through stages • Update program counter Data Data Memory memory memory Addr , Data valE CC CC ALU ALU Execute Bch aluA , aluB valA , valB srcA , srcB Decode A A B B dstA , dstB M M Register Register Register Register file file file file E E icode , ifun valP rA , rB valC Instruction PC Instruction PC memory increment Fetch memory increment PC

newPC SEQ Stages PC valE , valM Write back valM Fetch • Read instruction from instruction memory Decode • Read program registers Execute • Compute value or address Memory • Read or write data Write Back • Write program registers PC • Update program counter Data Data Memory memory memory Addr , Data valE CC CC ALU ALU Execute Bch aluA , aluB valA , valB srcA , srcB Decode A A B B dstA , dstB M M Register Register Register Register file file file file E E icode , ifun valP rA , rB valC Instruction PC Instruction PC memory increment Fetch memory increment PC

Optional Optional D icode 5 0 rA rB ifun rA rB valC Instruction Decoding Instruction Format • Instruction byte icode:ifun • Optional register byte rA:rB • Optional constant word valC

Instructor: Erol Sahin