Final Exam Notes

Syntactical error possibilities:
operands consistent with operation? (e.g., byte/long)
parameters allocated correctly? (order)
local variables allocated correctly?
storage space allocated on the stack for local vars?
loops have unconditional jumps back to beginning?
operands correct? (e.g., memory to memory)
local/parameter memory access consistency
jumping to correct labels? (e.g., begin while, not repeating init while)
pointer operations correct? (e.g., gets pointer address, not pointer value)
operands for add/sub/cmp correct? (e.g., addl %eax,$5)
void * dereference without cast?

Strategies:
comment code
read ahead and find out use of registers
no assembler-catchable errors


Exit code:

movl $1,%eax
movl $0,%ebx
int $0x80

Function entrance code:
//functionFoo(int *paramOne, char *paramTwo);
oldEbp = 0
retAddr = oldEbp + 4
paramOne = retAddr + 4
paramTwo = paramOne + 4
localOne = oldEbp - 4
localTwo = localOne - 4

pushl %ebp
movl %esp, %ebp

addl $localTwo,%esp

pushl %ebx
pushl %ecx
pushl %edx

... function body ...

popl %edx
popl %ecx
popl %ebx

subl $localTwo,%esp

popl %ebp

ret

Function call:
// functionFoo(int *paramOne, char *paramTwo);

pushl paramTwo(%ebp) // if paramTwo is char *
pushl paramOne(%ebp)
call functionFoo
addl $8,%esp


Moving data:
// if X_size defined statically
X_size: .fill 4, 1, 0xff // X_size is a memory address
...
movl X_size,%eax // eax is 0xffffffff (go to the address and get the value)
movl $X_size,%eax // eax is 0x804909c (treat the address as a literal)
// if X_size defined as a value
X_size = 4 // X_size is a value
...
movl X_size,%eax // eax is 0x8049090 (go to the value's addr and get data)
movl X_size+3,%eax
movl $X_size,%eax // eax is 4 (treat the value as a literal)

Incrementing pointers:

q = p + 1 // q is int32 *q; p is int32 *p
movl p(%ebp),%eax
addl $4,%eax //4 because sizeof(int32) = 4

Popping an element into memory:
popl Y_f1+X_f1(%eax)

Comparing byte:
cmpb $'\n',a+X_ch(%ebp)

$L1-2 evaluates L1-2 first