CS 301 Fall 2002 Control Structures Slide Set 5 9/21/2018

CS 301 Fall 2001 – Chapter 7 Slides by Prof. Hartman, following “IBM PC Assembly Language Programming” by Peter Abel 9/21/2018

The Stack Three main uses Saving return addresses for subroutines Passing data to subroutines Temporarily saving contents of registers so the program can use those registers for computation. 9/21/2018

The Stack 2 SS contains the address of the beginning of the stack, SP contains the size of the stack (and thus points to one address past the end of the stack) The stack begins storing data at the highest location in the segment and stores data downward through memory using PUSH and POP (and other) instructions. 9/21/2018

Stack Instructions PUSH, POP to and from general register, segment register, or memory PUSHA, POPA – save and restore contents of all general purpose registers (16 bytes) PUSHF, POPF – save and restore contents of the flags PUSHAD, POPAD – save and restore contents of all extended registers. (32 bytes) 9/21/2018

Instruction Execution and Addressing Processor steps in executing an instruction Fetch next instruction from memory and place it in instruction queue Decode the instruction: calculate addresses for memory references, deliver data to the ALU, increment IP Execute the instruction: perform the operation, store results in register or memory, set any required flags. 9/21/2018

Address types Short – Same segment, one byte offset, -128 to +127 Near – Same segment, two byte (80286 and earlier) or four byte (80386 and later) offset. Far – Different segment 9/21/2018

Branching Instructions JMP can jump to Short, Near, or Far addresses Jxx can jump to Short or Near (80386+) addresses LOOP can jump to Short addresses CALL can jump to Near or Far addresses 9/21/2018

Short Jumps If the label is before the jump (jumping back) NASM will automatically choose a Short jump if possible. If the label is after the jump (jumping forward) NASM will always use a Near jump, unless you specify jmp short label 9/21/2018

NASM labels Labels beginning with a period are “local” labels – they are associated with the most recent non-local label. 9/21/2018

Converting high-level control structures – if/else if ( condition ) { // body of then_block } else { // body of else_block In C is roughly equivalent to the following assembly code. Note the use of local labels. ; code to set flags based on condition jxx .else_block ; select xx to branch if false ; code for body of then_block jmp .endif .else_block: ; code for body of else_block .endif: 9/21/2018

Converting high-level control structures – while while ( condition ) { // body of loop } In C is roughly equivalent to the following assembly code. Note the use of local labels. .while: ; code to set flags based on condition jxx .endwhile ; select xx so that branches if false ; body of loop jmp .while .endwhile: 9/21/2018

Converting high-level control structures – do/while // body of loop } while ( condition ) In C is roughly equivalent to the following assembly code. Note the use of local labels. .do: ; code for body of loop ; code to set flags based on condition jxx .do ; select xx so branches if true 9/21/2018

Converting high-level control structures – for for(int i=0;i<10;++i) { // body of loop } In C is roughly equivalent to the following assembly code. Note the use of local labels. mov ecx, 10 .for: ; code for body of loop dec ecx jnz .for 9/21/2018

LOOP instruction LOOP label LOOPE/LOOPZ label LOOPNE/LOOPNZ label Decrements ecx (or cx in 16-bit mode) and branches to label unless ecx is then zero. LOOPE/LOOPZ label Adds condition that ZF=1. LOOPNE/LOOPNZ label Adds condition that ZF=0. 9/21/2018

Converting high-level control structures – for for(int i=0;i<10;++i) { // body of loop } In C is roughly equivalent to the following assembly code. Note the use of local labels. mov ecx, 10 .for: ; code for body of loop loop .for ; 9/21/2018

CALL and RET CALL proc_name RET [n] Pushes IP, sets IP to offset of proc_name (and clears processor’s prefetch instruction queue) RET [n] Pops IP (and clears processor’s prefetch instruction queue) Possibly “pops” n arguments from the stack 9/21/2018

Passing parameters Can pass parameters by reference (address) or value. Can pass parameters in registers or on stack. Examples using registers: regpassing.asm 9/21/2018

Passing parameters on the stack 1 Push parameters on the stack before the CALL instruction Procedure doesn’t pop them off, it accesses them directly on the stack: Avoids having to pop off return address then put it back on Allows using the parameter multiple times Need to use indirect addressing Examples using stack: stackpassing.asm 9/21/2018

Indirect Addressing Can add registers and/or constants and/or a location and get at what is located in the result MOV eax,[data] MOV eax,[ebx] MOV eax,[data+ebx] MOV eax,[ebx+2] MOV eax,[ebx*8+esp+4] 9/21/2018