vln37/libasm

Assembly yourself!

libasm

Some notes i aggregated while studying

Registers, what is each used for

Parameters

In order, 1st to 6th parameter.

rdi, rsi, rdx, rcx, r8, r9

subsequent ones stored on the LIFO stack

Data Registers

RAX, RBX, RCX, RDX

• AX - ACCUMULATOR register, return values
• BX - BASE register
• CX - COUNT register, stores loop counts
• DX - DATA register

Pointer Registers

RIP, RSP, RBP

• IP - INSTRUCTION pointer, you cannot access this
• SP - STACK pointer
• BP - BASE pointer

Index Registers

RSI, RDI

• SI - SOURCE index, used in string index operations
• DI - DESTINATION index, used in string index operations

Caller vs Callee

All caller registers must b saved in a safe place before calling a function

register	byte 0-3	bytes 0-1	byte 0	who saves
quadword	doubleword	word	byte	-
%rax	%eax	%ax	%al	Caller
%rbx	%ecx	%bx	%bl	Called Function
%rdi	%edi	%di	%dil	Caller (arg1)
%rsi	%esi	%si	%sil	Caller (arg2)
%rdx	%edx	%dx	%dl	Caller (arg3)
%rcx	%ecx	%cx	%cl	Caller (arg4)
%rsp	%esp	%sp	%spl	Stack ptr ?
%rbp	%ebp	%bp	%bpl	Called Function
%r8	%r8d	%r8w	r8b	Caller (arg5)
%r9	%r9d	%r9w	r9b	Caller (arg6)
%r10	%r10d	%r10w	r10b	Caller
%r11	%r11d	%r11w	r11b	Caller
%r12	%r12d	%r12w	r12b	Called Function
%r13	%r13d	%r13w	r13b	Called Function
%r14	%r14d	%r14w	r14b	Called Function
%r15	%r15d	%r15w	r15b	Called Function

Debugging tips

• gdb a.out
• tui enable
• break (func, asm label, symbols)
• layout regs or 'i r general' or 'disp /s $eax'
• dereference value in gdb x/d $r12, x/s $r12 x/format $r12
• run