MIPS ISA: Instruction Set Architecture & Examples

1. CSCE 212Chapter 2: Instruction Set Architecture Instructor: Jason D. Bakos

2. Lecture Outline • Instruction Set Architectures • MIPS ISA • MIPS Instructions, Encoding, Addressing Modes • MIPS Assembly Examples • SPIM • Procedure Calling Conventions • I/O

3. Instruction Set Architecture

4. Instruction Set Architecture • Instruction Set Architecture: • abtraction that hides the low-level details of a processor from the user • the interface between the hardware and software • everything you need to know to “use” the processor: • instruction set • instruction representations • addressing modes • etc… • “Families” of processors are defined by their ISA: • Sun Sparc • Intel IA-32 • MIPS • IBM 360 • Motorola/IBM PowerPC

5. ISAs Today

6. Processor Classes

7. MIPS ISA • 100 million MIPS processors manufactured in 2002 • MIPS processors used in: • Products from ATI, Broadcom, NEC, Texas Instruments, Toshiba • SGI workstations • Series2 TiVo • Windows CE devices • Cisco/Linksys routers • Nintendo 64 • Sony Playstation 1, PS2 (Emotion), PSP • Cable boxes • Competes against XScale/ARM for cell phones • John L. Hennessy (Stanford, 1981) • 1984: MIPS Computer Systems • R2000 (1985), R3000 (1988), R4000 (64-bit, 1991) • SGI acquisition (1992) => MIPS Technologies • Transition to licensed IP: MIPS32 and MIPS64 (1999) • “Heavyweight” embedded processor

8. Lecture Outline • Instruction Set Architectures • MIPS ISA • MIPS Instructions, Encoding, Addressing Modes • MIPS Assembly Examples • SPIM • Procedure Calling Conventions • I/O

9. MIPS Microarchitecture

10. RISC vs. CISC • Design “philosophies” for ISAs: RISC vs. CISC • CISC = Complex Instruction Set Computer • RISC = Reduced Instruction Set Computer • Tradeoff: • Execution time = instructions per program * cycles per instruction * seconds per cycle • Problems with CISC: • Compilers • Off-chip memory references • Complex control, unbalanced instruction set made parallelizing difficult • MIPS is the first implementation of a RISC architecture

11. RISC vs. CISC • MIPS R2000 ISA • Designed for use with high-level programming languages • Easy for compilers • Example: mapping IA32 instruction CRC32 (accumulate CRC32 value) • Balance amount of work per instruction (pipelining) • Load-store machine • Force user to minimize off-chip accesses • Fixed instruction width (32-bits), small set of uniform instruction encodings • Reduce implementation complexity

12. Lecture Outline • Instruction Set Architectures • MIPS ISA • MIPS Instructions, Encoding, Addressing Modes • MIPS Assembly Examples • SPIM • Procedure Calling Conventions • I/O

13. MIPS Instruction Types • MIPS instructions fall into 5 classes: • Arithmetic/logical/shift/comparison • Control instructions (branch and jump) • Load/store • Other (exception, register movement to/from GP registers, etc.) • Three instruction encoding formats: • R-type (6-bit opcode, 5-bit rs, 5-bit rt, 5-bit rd, 5-bit shamt, 6-bit function code) • I-type (6-bit opcode, 5-bit rs, 5-bit rt, 16-bit immediate) • J-type (6-bit opcode, 26-bit pseudo-direct address)

14. Partial MIPS Instruction Set (see Appendix. A) • Arithmetic R-type: add, addu, sub, subu • Arithmetic I-type: addi, addiu • Logical R-type: and, or, nor, xor • Logical I-type: andi, ori, xori • Compare R-type: slt, sltu • Compare I-type: slti, sltiu • Shift R-type: sll, sllv, srl, srlv, sra, srav • Load/Store I-type: lui, lw, lh, lhu, lb, lbu, sw, sh, sb • Branch I-type: • beq, bne, bgez, bgezal, bgtz, blez, blezal, bltz • Jump J-type: j, jal • Jump R-type: jr, jalr • OS support: syscall • Multiply/divide: mult, multu, div, divu • result held in 2 special registers (hi,lo) • Floating-point instructions

15. MIPS Registers • 32 x 32-bit general purpose integer registers • Some have special purposes • These are the only registers the programmer can directly use • $0 => constant 0 • $1 => $at (reserved for assembler) • $2,$3 => $v0,$v1 (expression evaluation and results of a function) • $4-$7 => $a0-$a3 (arguments 1-4) • $8-$15 => $t0-$t7 (temporary values) • Used when evaluating expressions that contain more than two operands (partial solutions) • Not preserved across function calls • $16-$23 => $s0->$s7 (for local variables, preserved across function calls) • $24, $25 => $t8, $t9 (more temps) • $26,$27 => $k0, $k1 (reserved for OS kernel) • $28 => $gp (pointer to global area) • $29 => $sp (stack pointer) • $30 => $fp (frame pointer) • $31 => $ra (return address, for branch-and-links) • Program counter (PC) contains address of next instruction to be executed

16. Design Considerations • Most arithmetic instructions have 3 operands simplifies the hardware • Limits the number of datapaths on the processor • Limiting to 32 registers speeds up register access time • For memories, smaller is faster • Influences clock cycle time

17. Arithmetic • Arithmetic (R-type) instructions add a,b,c a = b + c sub a,b,c a = b - c • C code: • f = (g + h) – (i + j) • to… add t0,g,h add t1,i,j sub f,t0,t1 • t0, t1, f, g, h, i, j must be registers

18. Registers • f, g, h, i, j in $s0, $s1, $s2, $s3, $s4 • To… add $t0,$s1,$s2 add $t1,$s3,$s4 sub $s0,$t0,$t1 • Similar instructions: • addu, subu • and, or, nor, xor • slt, sltu

19. Encoding R-type Instructions • ADD $2, $3, $4 • R-type A/L/S/C instruction • Opcode is 0’s, rd=2, rs=3, rt=4, func=100000 • 000000 00011 00100 00010 00000 100000 • 0000 0000 0110 0100 0001 0000 0010 0000 • 0000 0000 0110 0100 0001 0000 0010 0000 • 00641020

20. Immediate Instructions • Second operand is a 16-bit immediate • Signed (-32,768 to 32,767) or unsigned (0 to 65,535) • Encoded with I-type • addi $s0, $t0, -4 • Similar I-type instructions: • addiu • andi, ori, xori • lui

21. Encoding I-Type Arithmetic/Logical/Compare • ADDI $2, $3, 12 • I-type A/L/S/C instruction • Opcode is 001000, rs=3, rt=2, imm=12 • 001000 00011 00010 0000000000001100

22. Load Upper Immediate • Need more than 16 bits? • Example: • Initialize register $t0 with 1234567816 • lui $t0, 1234 • addi $t1, $0, 5678 • or $t0, $t0, $t1

23. Shift Instructions • Shift left-logical: • 001010012 by 210 => 101001002 • Multiply 4110 by 2210 = 16410 • Shift right-logical: • 001010012 by 210 => 000010102 • Divide 4110 by 2210 (round down) = 1010 • Shift right-arithmetic • 111101012 by 210 => 111111012 • Divide -1110 by 2210 (round down) = -310 • Amount (0-31) is encoded in SHAMT field for SLL, SRL, SRA • Held in a register (rs field) for SLLV, SRLV, SRAV

24. Load and Store • Memory units: • word (32 bits, 4 bytes) • halfword (16 bits, 2 bytes) • byte (8 bits) • Assume f, g, h, i, j are stored as words and contiguously • la $t2, f • lw $s1,4($t2) • lw $s2,8($t2) • lw $s3,12($t2) • lw $s4,16($t2) • … • sw $s0,0($t2) • Similar instructions: • lh, lhu, lb, lbu • sh, sb

25. Encoding I-Type Load/Store • SW $2, 128($3) • I-type memory address instruction • Opcode is 101011, rs=00011, rt=00010, imm=0000000010000000 • 101011 00011 00010 0000000010000000

26. Branch Instructions • Branch and jump instructions are required for program control • if-statements • loops • procedure calls • Unconditional branch • b <label> • Conditional branch • beq, bgez, bgezal, bgtz, blez • “and-link” variants write address of next instruction into $31 (only if branch is taken) • Branch targets are 16-bit immediate offset (offset in words)

27. Encoding I-Type Branch • BEQ $3, $4, 4 • I-type conditional branch instruction • Opcode is 000100, rs=00011, rt=00100, imm=4 (skips next 4 instructions) • 000100 00011 00100 0000000000000100 • Note: • bltz, bltzal, bgez, bgezal all have opcode 1, func in rt field

28. Jump Instructions • Unconditional branch • Two types: R-type and J-type • JR $31 • JALR $3 • R-type jump instruction • Opcode is 0’s, rs=3, rt=0, rd=31 (by default), func=001001 • 000000 00011 00000 11111 00000 001001 • J 128 • J-type pseudodirect jump instruction • Opcode is 000010, 26-bit pseudodirect address is 128/4 = 32 • 000010 00000000000000000000100000

29. MIPS Addressing Modes • MIPS addresses register operands using 5-bit field • Example: ADD $2, $3, $4 • MIPS addresses branch targets as signed instruction offset • relative to next instruction (“PC relative”) • in units of instructions (words) • held in 16-bit offset in I-type • Example: BEQ $2, $3, 12 • Immediate addressing • Operand is help as constant (literal) in instruction word • Example: ADDI $2, $3, 64

30. MIPS Addressing Modes (con’t) • MIPS addresses jump targets as register content or 26-bit “pseudo-direct” address • Example: JR $31, J 128 • MIPS addresses load/store locations • base register + 16-bit signed offset (byte addressed) • Example: LW $2, 128($3) • 16-bit direct address (base register is 0) • Example: LW $2, 4092($0) • indirect (offset is 0) • Example: LW $2, 0($4)

31. Integer Multiply and Divide • mult $2, $3 • result in hi (32 bits) and lo (32 bits) • mul $2, $3, $4 is psuedo (low 32 bits) • madd $2, $3 – multiply and accumulate in hi and lo • div $2, $3 • quotient in lo and reminder in hi • div $2, $3, $4 is psuedo (quotient)

32. Pseudoinstructions • Some MIPS instructions don’t have direct hardware implementations • Ex: abs $2, $3 • Resolved to: • bgez $3, pos • sub $2, $0, $3 • j out • pos: add $2, $0, $3 • out: … • Ex: rol $2, $3, $4 • Resolved to: • addi $1, $0, 32 • sub $1, $1, $4 • srlv $1, $3, $1 • sllv $2, $3, $4 • or $2, $2, $1

33. Lecture Outline • Instruction Set Architectures • MIPS ISA • MIPS Instructions, Encoding, Addressing Modes • MIPS Assembly Examples • SPIM • Procedure Calling Conventions • I/O

34. Complex Arithmetic Example z=(a*b)+(c/d)-(e+f*g); lw $s0,a lw $s1,b mult $s0,$s1 mflo $t0 lw $s0,c lw $s1,d div $s0,$s1 mflo $t1 add $t0,$t0,$t1 lw $s0,e lw $s1,f lw $s2,g mult $s1,$s2 mflo $t1 add $t1,$s0,$t1 sub $t0,$t0,$t1 sw $t0,z

35. If-Statement if ((a>b)&&(c==d)) e=0; else e=f; lw $s0,a lw $s1,b bgt $s0,$s1,next0 b nope next0: lw $s0,c lw $s1,d beq $s0,$s1,yup nope: lw $s0,f sw $s0,e b out yup: xor $s0,$s0,$s0 sw $s0,e out: …

36. For Loop for (i=0;i<a;i++) b[i]=i; lw $s0,a li $s1,0 loop0: blt $s1,$s0,loop1 b out loop1: sll $s2,S1,2 sw $s1,b($s2) addi $s1,$s1,1 b loop0 out: …

37. Pre-Test While Loop while (a<b) { a++; } lw $s0,a lw $s1,b loop0: blt $s0,$s1,loop1 b out loop1: addi $s0,Ss0,1 sw $s0,a b loop0 out: …

38. Post-Test While Loop • do { a++; • } while (a<b); lw $s0,a lw $s1,b loop0: addi $s0,$s0,1 sw $s0,a blt $s0,$s1,loop0 …

39. Complex Loop for (i=0;i<n;i++) a[i]=b[i]+10; li $2,$0 # zero out index register (i) lw $3,n # load iteration limit sll $3,$3,2 # multiply by 4 (words) la $4,a # get address of a (assume < 216) la $5,b # get address of b (assume < 216) j test loop: add $6,$5,$2 # compute address of b[i] lw $7,0($6) # load b[i] addi $7,$7,10 # compute b[i]=b[i]+10 add $6,$4,$2 # compute address of a[i] sw $7,0($6) # store into a[i] addi $2,$2,4 # increment i test: blt $2,$3,loop # loop if test succeeds

40. Lecture Outline • Instruction Set Architectures • MIPS ISA • MIPS Instructions, Encoding, Addressing Modes • MIPS Assembly Examples • SPIM • Procedure Calling Conventions • I/O

41. SPIM

42. SPIM • ASM file must be edited with text editor • Must have main label • Must jr $31 at end • Use .data and .text to specify sections • Load source file into SPIM • Run, step, or use breakpoints • Appendix A is good reference • In-class example: ASCII to binary conversion

43. Example Code .data mystr: .asciiz "2887" .text main: addi $s0,$0,0 # initialize $s0 (current value) addi $s1,$0,0 # initialize $s1 (string index) addi $s3,$0,10 # initialize $s3 (value 10) loop: lb $s2,mystr($s1) # load a character from string beqz $s2,done # exit if it's the NULL character mul $s0,$s0,$s3 # multiply current value by 10 addi $s2,$s2,-48 # subtract 48 from character (convert to binary) add $s0,$s0,$s2 # add converted value to current value addi $s1,$s1,1 # add one to index b loop # loop done: jr $31 # return to OS

44. Lecture Outline • Instruction Set Architectures • MIPS ISA • MIPS Instructions, Encoding, Addressing Modes • MIPS Assembly Examples • SPIM • Procedure Calling Conventions • I/O

45. Procedures • JAL, JALR, and BGEZAL are designed to call subroutines • Return address is linked into $31 ($ra) • Need to: • save the return address on a stack to save the return address • save the state of the callee’s registers on a stack • have a place for arguments • have a place for return value(s)

46. Memory Allocation

47. The Stack • Stack is designed to hold variable-sized records • Stack grows down • Normally the old $fp must be stored in the AR to pop • Don’t need $fp for fixed-sized AR’s

48. A Simple Procedure Calling Convention • Caller: • Place arguments in $a0 - $a3 (limit to 4) • Jump-and-link or branch-and-link to subroutine • Callee: • Pushes an activation record onto the stack (decrement $sp) • Save the return address ($ra) on the AR • Save registers $s0 - $s7 on the AR • Perform computation • Save return values to $v0 and $v1 • Restore $s0 - $s7 • Restore $ra • JR $ra • Caller: • Reads $v0 and $v1 and continues

49. Notes • This convention: • Limited to 4 arguments and 2 return values (bad!) • Doesn’t save $t0 - $t9, $v0 - $v1, and $a0 - $a3 (bad!) • Doesn’t allow (variable-size) space on the AR for argument list (saves regs) • Doesn’t allow (variable-size) space on the AR for callee’s local variables (bad!) • Doesn’t allow space on the AR for return value (saves regs) • Fixed AR size (good!) • Doesn’t require the caller to prepare and/or teardown the AR (good!)

50. Stack Example