CPSC 161 Lecture 3

1. CPSC 161Lecture 3 Prof. L.N. Bhuyan http://www.cs.ucr.edu/~bhuyan/

2. Review of MIPS Instruction Formats • simple instructions all 32 bits wide • very structured, no unnecessary baggage • only three instruction formats op rs rt rd shamt funct R I J op rs rt 16 bit address op 26 bit address

3. MIPS Instructions: R-format I- format I-format <- R-format <- J-format <- R-format <- J-format

4. Assembly Operands: Registers • Naming of 32 MIPS registers: instead of r0, r1, …, r31, use • $s0, $s1, … for registers corresponding to C variables • $t0, $t1, … for registers corresponding to temporary variables • Will explain mapping convention later of $s0, $s1, … , $t0, $t1, … , to r0, r1, … • Note: whereas C declares its variables (e.g., int fahr), Assembly operands (registers) are fixed and not declared

5. Policy of Use Conventions

6. Role of Registers vs. Memory • What if more variables than registers? • Compiler tries to keep most frequently used variables in registers • Writing less common to memory: spilling • Why not keep all variables in memory? • Smaller is faster: registers are faster than memory • Registers more versatile: • MIPS arithmetic instruction can read 2, operate on them, and write 1 per instruction • MIPS data transfer only read or write 1 operand per instruction, and no operation

7. Compilation using Registers • Compile by hand using registers:f = (g + h) - (i + j); Register Allocations:f: $s0, g: $s1, h: $s2, i: $s3, j: $s4 • MIPS Instructions: add $s0,$s1,$s2 # $s0 = g+h add $t1,$s3,$s4 # $t1 = i+j sub $s0,$s0,$t1 # f=(g+h)-(i+j)

8. MIPS Instruction Encoding Examples of some Opcodes: Instruction Format Opcode shamt funct Add R 0 0 32 Sub R 0 0 34 Shift (by 4) R 0 4 0 Add (imm) I 8 n.a n.a Lw (load word) I 35 n.a n.a Sw (store word) I 43 n.a n.a

9. Data Transfer Instruction: Memory to Reg • Load: moves data from memory to register • Syntax: 1) operation name 2) register to be loaded 3) constant and register to access memory • MIPS name, lw for load word: • Example: lw $t0, 8($s3) Called “offset” Called “base register” or “base address register” or “base address”

10. Compilation when Operand is in Memory Q: Compile by hand using registers:g = h + A[300];g:$s1, h:$s2,$s3:starting (base) address of array A • Since A[300] is in memory, 1st transfer from memory to (temporary) register: lw $t0,300($s3) # Adds 300 to $s3 to select A[300], puts into $t0 lw $t0,1200($s3) # For byte addressable machines 300x4 • Next add it to h and place in g add $s1,$s2,$t0 # $s1= h+A[300] HW: Compile A[300] = h + A[300]

11. Tanslating to MIPS Machine Language • From the instruction set, Opcode for Lw is 35. Opcode for add is 0 with funct 32. • From register assignment table, t0=8, s1=17, s2=18 and s3=19. • Instruction consists of op=5 bits, rs=5bits, rt=5bits, rd=5bits, shamt=5bits and funct=6bits for R format and address=16 bits instead of rd,shamt and funct for I format: total=32 bits Assembly language lw $t0, 1200($s3) and add $s1,$s2,$t0 translate to: --------------------------------------------------------------------------- op | rs | rt | rd | address/shamt | funct | 35 | 19 | 8 | 1200 | 0 | 18 | 8 | 17 | 0 | 32 |

12. Compile with variable index • What if array index not a constant?g = h + A[i]; • g:$s1, h:$s2, i:$s4, $s3:base address ofA • To load A[i] into a register, first turn i into a byte address; multiply by 4 • How multiply using adds? • i + i = 2i, 2i + 2i = 4i add $t1,$s4,$s4 # $t1 = 2*i add $t1,$t1,$t1 # $t1 = 4*i

13. Compile with variable index, con’t • Next add to base of A: add $t1,$t1,$s3 #$t1=address of#A[i] (4*i+$s3) • Now load A[i] into a temporary register: lw $t0,0($t1) # Temp $t0 = A[i] • Finally add to h and put sum in g: add $s1,$s2,$t0 # g = h + A[i]

14. MIPS arithmetic instructions Instruction Example Meaning Comments add add $1,$2,$3 $1 = $2 + $3 3 operands; subtract sub $1,$2,$3 $1 = $2 – $3 3 operands; add immediate addi $1,$2,100 $1 = $2 + 100 + constant; add unsigned addu $1,$2,$3 $1 = $2 + $3 3 operands; subtract unsigned subu $1,$2,$3 $1 = $2 – $3 3 operands; add imm. unsign. addiu $1,$2,100 $1 = $2 + 100 + constant; multiply mult $2,$3 Hi, Lo = $2 x $3 64-bit signed product multiply unsigned multu$2,$3 Hi, Lo = $2 x $3 64-bit unsigned product divide div $2,$3 Lo = $2 ÷ $3, Lo = quotient, Hi = remainder Hi = $2 mod $3 divide unsigned divu $2,$3 Lo = $2 ÷ $3, Unsigned quotient & remainder Hi = $2 mod $3 Move from Hi mfhi $1 $1 = Hi Used to get copy of Hi Move from Lo mflo $1 $1 = Lo Used to get copy of Lo Which add for address arithmetic? Which add for integers?

15. MIPS logical instructions Instruction Example Meaning Comment and and $1,$2,$3 $1 = $2 & $3 3 reg. operands; Logical AND or or $1,$2,$3 $1 = $2 | $3 3 reg. operands; Logical OR xor xor $1,$2,$3 $1 = $2 Å $3 3 reg. operands; Logical XOR nor nor $1,$2,$3 $1 = ~($2 |$3) 3 reg. operands; Logical NOR and immediate andi $1,$2,10 $1 = $2 & 10 Logical AND reg, constant or immediate ori $1,$2,10 $1 = $2 | 10 Logical OR reg, constant xor immediate xori $1, $2,10 $1 = ~$2 &~10 Logical XOR reg, constant shift left logical sll $1,$2,10 $1 = $2 << 10 Shift left by constant shift right logical srl $1,$2,10 $1 = $2 >> 10 Shift right by constant shift right arithm. sra $1,$2,10 $1 = $2 >> 10 Shift right (sign extend) shift left logical sllv $1,$2,$3 $1 = $2 << $3 Shift left by variable shift right logical srlv $1,$2, $3 $1 = $2 >> $3 Shift right by variable shift right arithm. srav $1,$2, $3 $1 = $2 >> $3 Shift right arith. by variable

16. MIPS data transfer instructions Instruction Comment SW 500(R4), R3 Store word SH 502(R2), R3 Store half SB 41(R3), R2 Store byte LW R1, 30(R2) Load word LH R1, 40(R3) Load halfword LHU R1, 40(R3) Load halfword unsigned LB R1, 40(R3) Load byte LBU R1, 40(R3) Load byte unsigned LUI R1, 40 Load Upper Immediate (16 bits shifted left by 16) LUI R5 R5 0000 … 0000

17. Section 2.6 MIPS decision instructions • Decision instruction in MIPS: • beq register1, register2, L1 • beq is “Branch if (registers are) equal” Same meaning as (using C): if (register1==register2) go to L1 • Complementary MIPS decision instruction • bne register1, register2, L1 • bne is “Branch if (registers are) not equal” Same meaning as (using C): if (register1!=register2) go to L1 • Called conditional branches

18. (false) i != j (true) i == j f=g+h f=g-h i == j? Compiling C if into MIPS: Summary • Compile by hand if (i == j) f=g+h; else f=g-h; Mapping f: $s0, g: $s1, h: $s2, i: $s3, j: $s4 beq $s3,s4, True # branch i==j sub $s0,$s1,$s2 # f=g-h(false) j Exit # go to ExitTrue: add $s0,$s1,$s2 # f=g+h (true)Exit: • Note: Compiler supplies labels, branches not found in HLL code; often it flips the condition to branch to false part C MIPS

19. Loops in C/Assembly: Summary Loop: g = g + A[i]; i = i + j; if (i != h) goto Loop; (g,h,i,j:$s1,$s2,$s3,$s4 : base of A[]:$s5) Loop: add $t1,$s3,$s3 #$t1= 2*i add $t1,$t1,$t1 #$t1= 4*i add $t1,$t1,$s5 #$t1=addr A lw $t1,0($t1) #$t1=A[i]add $s1,$s1,$t1 #g=g+A[i] add $s3,$s3,$s4 #i=i + j bne $s3,$s2,Loop# goto Loop # if i!=h C MIPS

20. op rs rt address Branch Addressing: PC-relative I • Conditional Branch:beq $t0,$t1,label • address just 16 bits (216), program too small! • Option: always add address to a register PC = Register + Branch address • Change register contents => bigger programs • Which register? • How use conditional branch? if-else, loops • Near current instruction => use PC as reg! • PC-relative addressing (PC+4) +/- 215 words 6 bits 5 bits 5 bits 16 bits

21. op 2 address 2500 Branch Addressing: Jumps, J format • j label # go to label • j has only one operand; add format • large address allows large programs • bright idea: address of instruction always multiple of 4 (instructions always words) => store number of word, save 2 bits • Example:j exit # exit = 10000 • PC = address * 4 + upper 4 bits of old PC J 6 bits 26 bits J

22. 80000 0 42 80004 80008 0 32 80012 80016 80020 0 32 80020 = 80004 + 4 + 3*4 Branch Addressing: PC-relative Example Loop: slt $t1,$zero,$a1 # t1=9,a1=5 beq $t1,$zero,Exit # no=>Exit add $t0,$t0,$a0 # t0=8,a0=4subi $a1,$a1,1 # a1=5 j Loop # goto LoopExit: add $v0,$t0,$zero # v0=2,t0=8 Set t1=1 if $zero < $a1 Address 0 5 9 0 4 9 0 3 8 4 8 0 8 5 5 -1 2 20000 8 0 2 0