CS208 Introduction to Computer Systems
Monday, 24 April 2023

+ Which order of arguments? Intel vs. AT&T notation.

    Intel
        mov rax, rbx    --  copies rbx into rax
        cmp rbx, rcx    --  subtract rbx - rcx
        jg somewhere    --  jump if result was > 0 (>=?)

    * AT&T
        mov %rax, %rbx    --  copies rax into rbx
        cmp %rbx, %rcx  --  subtract rcx - rbx
        jg somewhere    --  jump if result was > 0 (>=?)

+ q, l, w, b

    movq   --  copy a 64-bit quantity (rax)
    movl   --  copy a 32-bit quantity (eax)
    movw   --  copy a 16-bit quantity (ax)
    movb   --  copy an 8-bit quantity (al, ah)

+ Addressing modes

    mov %eax, %ebx                  --  register & register  
    mov $0x50, %ebx                 --  immediate & register
    mov (%rbx), %rax                --  indirect & register
        rbx has an address in it
        go to that address, grab the 8-byte quantity
            you find there and copy it into rax
        i.e., rbx is being used as a pointer
    mov (%rbx), %eax
        same, but grab 4 bytes of data

    mov 0x12(%rbx), %rax            -- indirect + offset & register
        rbx has an address
        go to (that address + 0x12)
            get the data, put it in rax

    mov (%rbx, %rcx, 1), %rax
        rbx has an address
        rcx is a counter/index
        go to (rbx + rcx*1)
            get the data, put it in rax

    mov 0x34(%rbx, %rcx, 1), %rax
        same, but (0x34 + rbx + rcx*1)

    mov (%rbx, %rcx, 4), %rax
        rbx has an address
        rcx is a counter/index
        go to (rbx + rcx*4)
            get the data, put it in rax

    mov 0x56(%rbx, %rcx, 4), %rax
        same, but (0x56 + rbx + rcx*1)

    mov (%rbx, %rcx), %rax
        same as (%rbx, %rcx, 1)

    mov 0x78(%rbx, %rcx), %rax
        same as 0x78(%rbx, %rcx, 1)

+ Special registers (much more on these later)

    rsp
    rip
    eflags

+ Some non-obvious-but-essential instructions

    cmp %rbx, %rcx   --  compute %rcx - %rbx, set eflags accordingly
    test %rbx, %rcx
    lea

+ Try this at godbolt.org

- What does a C loop look like in asm?
- What does a C if/else look like?
- What does a loop over an array look like?

- Harder: explore function-calling