Computer Organization CS224 Fall 2012 Lessons 9 and 10

 MIPS procedure call instruction: jalProcedureAddress#jump and link  Saves PC+4 in register $ra to have a link to the next instruction, for the procedure return; jumps to target addr.  Machine format (J format):  Procedure return copies return address to PC: jr$ra#return to callpoint+4;  Instruction format (R format): Instructions for Accessing Procedures 0x03 26 bit address x08 §2.8 Supporting Procedures in Computer Hardware

Six Steps in Execution of a Procedure 1. Main routine (caller) places parameters in a place where the procedure (callee) can access them $a0 - $a3 : four argument registers 2. Caller transfers control to the callee ( jal Dest) 3. Callee acquires the storage resources needed 4. Callee performs the desired task 5. Callee places the result value in a place where the caller can access it $v0 - $v1 : two value registers for result values 6. Callee returns control to the caller ( jr $ra) $ra : one return address register to return to the point of origin

Register Usage  $a0 – $a3: arguments (reg’s 4 – 7)  $v0, $v1: result values (reg’s 2 and 3)  $t0 – $t9: temporaries (reg’s 8-15, 24-25) l Can be overwritten by callee  $s0 – $s7: saved (reg’s 16-23) l Must be saved/restored by callee !  $gp: global pointer for static data (reg 28)  $sp: stack pointer (reg 29)  $fp: frame pointer (reg 30)  $ra: return address (reg 31)  $gp, $sp, $fp, $ra must be saved/restored by callee !

MIPS Register Convention NameRegister Number UsagePreserve on call? $zero0constant 0 (hardware)n.a. $at1reserved for assemblern.a. $v0 - $v12-3returned valuesno $a0 - $a34-7argumentsno $t0 - $t78-15temporariesno $s0 - $s716-23saved valuesyes $t8 - $t924-25temporariesno $k0 - $k126-27reserved for op systemn.a. $gp28global pointeryes $sp29stack pointeryes $fp30frame pointeryes $ra31return addr (hardware)yes

Stack Usage  Both caller and callee use a stack – a last-in-first-out queue low addr high addr $sp  One of the general registers, $sp ( $29 ), is used to address the stack (which “grows” down, from high address to low address) l add data onto the stack – push $sp = $sp – 4 put data on stack at new $sp l remove data from the stack – pop get data from stack at $sp $sp = $sp + 4 top of stack

Leaf Procedure Example  C code: int leaf_example (int g, h, i, j) { int f; f = (g + h) - (i + j); return f; } l Arguments g, …, j in $a0, …, $a3 l f in $s0 (hence, need to save $s0 on stack) l Return result in $v0

Leaf Procedure Example  MIPS code: leaf_example: addi $sp, $sp, -4 sw $s0, 0($sp) add $t0, $a0, $a1 add $t1, $a2, $a3 sub $s0, $t0, $t1 add $v0, $s0, $zero lw $s0, 0($sp) addi $sp, $sp, 4 jr $ra Note: it can be done in 4 lines (2 add, sub, jr) if $s0 is not used Save $s0 on stack Procedure body Restore $s0 Result to$v0 for return Return

Stacking of Subroutine Environments  Procedures are often nested, there are multiple calls and returns  C is a “leaf” procedure, but B is a nested (non-leaf) procedure  The stack grows downward at each new call, shrinks upward at each return A: A CALL B B: A B CALL C A B C C: RET A B A

Non-Leaf Procedures  Procedures that call other procedures  For nested call, caller needs to save on the stack: l Its return address l Any arguments and temporaries it still needs after the call (which are not in saved registers)  Restore from the stack after the call  As with leaf procedures, callee also needs to save on the stack any “saved” registers, & restore them at end

Non-Leaf Procedure Example  C code: int fact(int n) { if (n < 1) return (1); else return n * fact(n - 1); } l Argument n in $a0 l Return result in $v0

Non-Leaf Procedure Example  MIPS code: fact:addi $sp, $sp, -8 # adjust stack for 2 items sw $ra, 4($sp) # save return address sw $a0, 0($sp) # save argument slti $t0, $a0, 1 # test for n < 1 beq $t0, $zero, L1 addi $v0, $zero, 1 # if so, result is 1 addi $sp, $sp, 8 # pop 2 items from stack jr $ra # and return L1: addi $a0, $a0, -1 # else decrement n jal fact # recursive call lw $a0, 0($sp) # restore original n lw $ra, 4($sp) # and return address addi $sp, $sp, 8 # pop 2 items from stack mul $v0, $a0, $v0 # multiply to get result jr $ra # and return

Local Data on the Stack  Local data allocated by callee l e.g., C automatic variables  Procedure frame (aka activation record) l Used by some compilers to manage stack storage  The frame pointer ( $fp ) points to the first word of the frame of a procedure – providing a stable “base” register for the procedure

Using Registers to Implement Procedures  Caller l Save caller-saved registers $a0-$a3, $t0-$t9 (if their values need preserving) l Load arguments in $a0-$a3 (rest on stack above $fp) l Execute jal instruction  Callee Setup l Step 1: Allocate memory in frame ($sp = $sp - frame) l Step 2: Save callee-saved registers $s0-$s7, $fp, $ra (if their values will be changed) l Step 3: Create frame ($fp = $sp + frame size - 4)  Callee Return l Place return value in $v0 and $v1 l Restore any callee-saved registers ($fp, $ra,$s0-$s7…) l Pop stack ($sp = $sp + frame size) l Return using jr $ra

Calling Convention: Steps FP SP ra old FP $s0-$s7 FP SP First four arguments passed in registers Before call: Callee setup; step 1 FP SP Callee setup; step 2 ra old FP $s0-$s7 FP SP Callee setup; step 3 Adjust SP Save registers as needed Adjust FP

Memory Layout  Text: program code  Static data: global variables l e.g., static variables in C, constant arrays and strings l $gp is initialized to address , allowing +/- offsets into this segment  Dynamic data: heap l For structures that grow and shrink, e.g. linked lists, trees, etc l Allocate using malloc in C, (new in Java); free it with free in C (automatic garbage collection in Java)  Stack: storage of automatic variables (local to a procedure)

Character Data  Byte-encoded character sets l ASCII: 128 characters (= 7-bit) -95 graphic (i.e printable), 33 control l Latin-1: 256 characters (= 8-bit) -ASCII, +96 more graphic characters  Unicode: universal character set l Used in Java, C++ wide characters, … l Most of the world’s alphabets, plus symbols l UTF-16 is the default (16-bit encoding for each character) l UTF-32 is 32-bits per character l UTF-8 is variable length: 8-bits to handle ASCII, plus bits for other characters §2.9 Communicating with People

Byte/Halfword Operations  Could use bitwise operations—so why not? l String processing is a common case  MIPS byte/halfword load/store operations --lb rt, offset(rs) lh rt, offset(rs) l Sign extend to 32 bits in rt --lbu rt, offset(rs) lhu rt, offset(rs) l Zero extend to 32 bits in rt --sb rt, offset(rs) sh rt, offset(rs) l Store just rightmost byte/halfword

String Copy Example  C code (naive): l Null-terminated ASCII string void strcpy (char x[], char y[]) { int i; i = 0; while ((x[i]=y[i])!='\0') # copy x  y until null i += 1; } l Addresses of x, y in $a0, $a1 l i in $s0

String Copy Example  MIPS code: strcpy: addi $sp, $sp, -4 # push $s0 onto stack sw $s0, 0($sp) # add $s0, $zero, $zero # i = 0 L1: add $t1, $s0, $a1 # addr of y[i] in $t1 lbu $t2, 0($t1) # $t2 = y[i] add $t3, $s0, $a0 # addr of x[i] in $t3 sb $t2, 0($t3) # x[i] = y[i] beq $t2, $zero, L2 # exit loop if y[i] == 0 addi $s0, $s0, 1 # i = i + 1 j L1 # next iteration of loop L2: lw $s0, 0($sp) # pop $s0 from stack addi $sp, $sp, 4 # jr $ra # and return [Bottom testing & pointer addressing saves 2 lines from the loop. Not using s0 saves 4 other lines.]

 We'd like to be able to load a 32-bit constant into a register, for this we must use two instructions  a new "load upper immediate" instruction lui $t0,  Then must get the lower order bits right, use ori $t0, $t0, bit Constants §2.10 MIPS Addressing for 32-Bit Immediates and Addresses

Branch Addressing  Branch instructions specify l Opcode, two registers, target address l beq $t0, $s3, LoopEnd  Most branch targets are near branch l Forward or backward (max is ±2 15 ) oprsrtconstant or address 6 bits5 bits 16 bits  PC-relative addressing l Target address = PC + offset × 4 l PC already incremented by 4 by this time

 MIPS also has an unconditional branch instruction or jump instruction: j label#go to label Other Control Flow Instructions  Instruction Format (J Format): 0x02 26-bit address PC from the low order 26 bits of the jump instruction

Jump Addressing  Jump ( j and jal ) targets could be anywhere in text segment l Encode full address in instruction opaddress 6 bits 26 bits  Pseudo-Direct jump addressing l Target address = PC 31…28 : (address × 4)

Target Addressing Example  Loop code from earlier example l Assume Loop at location Loop: sll $t1, $s3, add $t1, $t1, $s lw $t0, 0($t1) bne $t0, $s5, Exit addi $s3, $s3, j Loop Exit: … 80024

Branching Far Away  What if the branch destination is further away than can be captured in 16 bits?  The assembler comes to the rescue – it inserts an unconditional jump to the branch target and inverts the condition beq$s0, $s1, L1_far becomes bne$s0, $s1, L2 jL1_far L2:

Addressing Mode Summary