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Assembly Cheat Sheet#

Register conventions#

  • General registers
  • Data registers: used for arithmetic, logical, and other operations
    • AX: the primary accumulator
    • BX: base register
    • CX: count register
    • DX: data register
  • Pointer registers
    • IP: Instruction Pointer
    • SP: Stack Pointer
    • BP: Base Pointer
  • Index registers
  • Control registers
  • Segment registers

General Purpose Registers#

Register Usage Description
rax/r0 accumulator used for input/output and most arithmetic instructions
return value holds function return value
syscall number holds syscall number argument for syscall instruction
rbx/r3 base for index addressing used as memory address base for indexed addressing
rcx/r1 counter for iteration used for holding loop count in loops
this storing this pointer in c++
rdx/r2 data used in arithmetic and I/O operations
rdi/r7 destination index used in stream operations e.g. stosb
rsi/r6 source index used in stream operations e.g. lodsb
rsp/r4 Stack pointer points to the top of the stack
rbp/r5 Frame pointer points to the frame’s base of stack
rn Numbered (n=8..15)
xmm0-15 128 bit floating point
rip instruction pointer points to next instruction to execute

Control Registers#

Register Description
CF carry flag condition code register
ZF zero flag condition code register
SF sign flag condition code register
OF overflow flag condition code register
  • Condition Code registers are single bit registers
  • implicitly set by arithmetic instructions
    • Ex: addl a,b where t is result value
    • CF set if unsigned overflow
    • ZF set if result is zero
    • SF set if result \< 0
    • OF set if signed overflow `(a>0 && b>0 && t\<0) || (a\<0 && b\<0 && t>=0)
  • explicitly set by compare instruction
    • cmp a,b uses sets same flags as a-b instruction but without setting destination

Segment Registers#

Register Description
CS Code segment
SS Stack segment
DS Data segment
ES Extra data segment
FS Points to Thread Information Block (TIB)
GS Extra data segment

Register Operands#

Operand Description
Immediate numeric constant value 0x234 or 48
Register general purpose register rax
Memory value stored at [reg + reg*scale + offset] address scale is 1, 2, 4, or 8 only
value stored at [offset] [2344]
value stored at [reg] [rax+4] or C analogue of *(rax+4)
value stored at [reg + offset] [rax+4] or C analogue of *(rax+4)
value stored at [reg + reg*scale] [rax+4] or C analogue of *(rax+4)
value stored at [reg + reg*scale + offset ] [rax+rbx*4] or C analogue of *(rbx*4+rax)
Register Data Suffix Definition instruction
8 bits (byte) b db
16 bits (word) w dw
32 bits (dword) l dd
64 bits (qword) ll ddq/do
float dd
double dq
extended precision dt
  • NOTE: dst can never be an immediate
  • NOTE: src and dst can’t both be memory operands
  • NOTE: some assemblers will legalize memory operands through simple algebraic transform
  • e.g. [rax*5] => [rax*4+rax]

Instructions#

Conditionals#

cmp op1, op2 -> mimics sub op1, op2 but only changes the zero and carry flag for comparing

  • Prefixes

  • j~ x: jump to x if ~

  • cmov~ x, y: conditional mov x, y if ~
  • setc~ x: set x to 1 if ~, x is 8 bit reg

  • Many suffixes

  • a: above, >

  • ae: above or equal, >=
  • b: below, \<
  • be: below or equal, \<=
  • e: equal, =
  • ne: not equal, !=

Calling Convention#

Linux/OS X#

  • function parameters passed in registers
  • ints/pointers: rdi, rsi, rdx, rcx, r8, r9
  • floats/doubles: xmm0, xmm1, xmm2, xmm3, xmm4, xmm5, xmm6, xmm7
  • additional parameters get pushed on the stack in reverse order (must be cleaned up by caller)
  • return values are stored in rax/xmm0 for int/float
  • stack pointer rsp must be aligned to 16-byte boundary before invocation
  • call instruction pushes the return address (8 bytes) which unaligns rsp
  • must manually align by pushing or subtracting 8 from rsp
  • callee-saved registers: rbp, rbx, r12, r13, r14, r15
extern putchar
mov rdi,'H' ; function parameter: one char to print
call putchar

Windows#

  • function parameters passed in registers: rcx, rdx, r8, r9
  • must allocate 32 bytes of shadow stack space
  • callee-saved registers: rbx, rbp, rdi, rsi, rsp, r12, r13, r14, r15, xmm6..xmm15
sub rsp,32+8; parameter area, and stack alignment
extern putchar
mov rcx,'H' ; function parameter: one char to print
call putchar
add rsp,32+8 ; clean up stack

Reference#

NASM Tutorial

Last update: January 27, 2022
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