Assembly Cheatsheet#

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#

Control Registers#

• 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 Operands#

• 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#

Common Instructions#

For more common instructions, check out the Stanford CS107 list

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
Some Assembly Required: An approachable introduction to assembly