Abdulhafeez Muhammad Al-Rabi Dr. Raed Al-Qadi Lecture 15

1. Abdulhafeez Muhammad Al-Rabi Dr. Raed Al-Qadi Lecture 15

2. CS536 – Introduction to Compilers Lecture 15 – MIPS Architecture .

3. MIPS Architecture and xspim : • Lecture examples and assignments use the MIPS R2000. • Simple, clean RISC computer • Easy to generate code • Similar to other modern computers • Run code on xspam (simulator for the R2000). • Advantages of a simulator : • Better debugging than bare machine. • Can generate code and test programs on many machines . • Cheaper than 30 R2000's • Disadvantages : • Slower • Miss the "extement" of debugging real hardware . • Documentation on xspam is at MACC.

4. R2000 • The R2000 is a Reduced Instruction Set (RISC) load-store architecture . • All operands must be explicitly loaded into a register before using them ( unlike the VAX). • Contains 32 general purpose (32-bit) registers. • Contains 32 single floating point (16 double floating point ) registers .

5. Pseudo-Instructions: • The assembler on MIPS ( and xspim) provide instructions that are not implemented in hardware . • Called pseudo-instructions. • Assembler translates pseudo-instructions to 1-3 MIPS instructions • eg • bge $3 , $4 ,label ;branch on R3 >= R4 to label • sub $1 , $3 , $4 ;$1 := $3 - $4 • bgez $1 label ;branch on R1 >=0 to label • Simplifies hardware since fewer instructions needed • Renaming instructions run faster . • Pseduo-instructions simplify programming • Bare machine is extremely unpleasant • Well-balanced hardware-software trade-off.

6. Registers: • The 32 general purpose registers have special uses (by convention) • $0 always contains a 0 (fixed by hardware) . • $1 is reserved for the assembler . • $v ($2) is used to return value from function. • $a0-$a3 (4 – 7) pass first 4 arguments to subroutines . • $t0-$t7 (8 – 15) are temporaries that may be destroyed by a call . • $s0-$s7 (16 – 23) are temporaries preserved across call. • $sp (29) is used as stack pointer . • $30 is used as a frame pointer . • $ra (31) is loaded with return address by a call instruction.

7. Load and Store : • Load and store have the following addressing mode : • Format Address • (reg) contains of registers int value of integer int(reg) int + reg • sym address of symbol sym +- int address +- int • sym +- int(reg) address +- int +- reg • lw R, address • R = address (must be multiple of 4) • lb R, address • R = address • la R, address • R = address • similarly for store .

8. Arithmetic: • add R1, R2, R3/const • R1 = R2 + R3 • sub R1, R2, R3/const • R1 = R2 – R3 • and R1, R2, R3/const • R1 = R2&R3 • Has the other operations : abs, div, mult, neg, nor, not, or, rem, rol, ror, sll, srl, xor • li R, Const • R = value

9. Branching: • j label • goto label • jr R • goto (R) • beq R1, R2, label • if(R1 == R2) goto label • Has Other tests : bge, bgeu, bgt, bgtu, blt, bltu, bne

10. Subroutines: • jal name • name() • Stores return address in $ra and jumbs to name • Return by executing • Jr $ra

11. Example: • Add numbers from 1 to 10 • li $t0, 0 ;t0 holds sum • li $t1, 1 ;t1 hold index • b test • loop: add $t0, $t0, $t1 ;sum+ = index • addi $t1, $t1, 1 ;index += 1 • test: ble $t1, 10, loop • Compute Length Of a string (strlen) • li $t0, 0 ;t0 holds sum • loop: lb $t1,0($a0) ;a0 holds a string • beqz $t1, done ;zero byte • add $t0, $t0, $t1 ;sum += *str • addi $0, $a0, 1 ;str ++;

12. Rest Of Course: • Will discuss code generation using the MIPS as the target machine • In each area , discuss semantics of language features and code to implement : • Subroutine calls • Local and global storage • Memory allocation • Arithmetic expressions • Flow control

13. The End