2021-12-20
This guide is part seven of the series, X86–64 Assembly Language Program.
This article is more of a reference guide for those learning x86–64 Assembly Language. I hope you find the information and links at the bottom helpful.
General-purpose registers — there are 16 general-purpose registers —
rax, rbx, rcx, rdx,
rbp, rsp, rsi, rdi,
r8, r9, r10, r11,
r12, r13, r14,
r15
data- section is used for declaring initialized data or
constantsbss- section is used for declaring non initialized
variablestext- section is used for coderdi- first argumentrsi- second argumentrdx- third argumentrcx- fourth argumentr8- fifth argumentr9- sixthThe first six integer or pointer arguments are passed in registers
rdi, rsi, rdx, rcx,
r8, r9. r10 is used as a static
chain pointer in the case of nested functions.
rax is used as the return value from a function.
The registers rbx, rbp, and
r12– r15 are preserved registers.
All other registers must be saved by the caller if it wishes to preserve its values.
ADD- integer addSUB- subtractMUL- unsigned multiplyIMUL- signed multiplyDIV- unsigned divideIDIV- signed divideINC- incrementDEC- decrementNEG- negateThe initial number must be stored in rax.
rax can be multiplied by a value in any of the other
registers. The result will be stored in rax.
JE - jump if equalJZ - jump if zeroJNE - jump if not equalJNZ - jump if not zeroJG - jump if the first operand is greater than
secondJGE - jump if the first operand is greater or equal to
secondJA - the same that JG, but performs an unsigned
comparisonJAE - the same that JGE, but performs an unsigned
comparisonThe fundamental data types are bytes, words, doublewords, quadwords, and double quadwords.
byte is eight bitsword is two bytesdoubleword is four bytesquadword is eight bytesdouble quadword is sixteen bytes (128 bits).Directives are commands that are part of the assembler syntax but are not related to the x86 processor instruction set. All assembler directives begin with a period (.).
The .DATA directive is used for setting values in
memory.
The syntax within the .DATA directive is
variable name define-directive
initial-value
There are five basic forms of the define directive −
+-----------+-------------------+-------------------+
| Directive | Purpose | Storage Space |
+-----------+-------------------+-------------------+
| DB | Define Byte | allocates 1 byte |
| DW | Define Word | allocates 2 bytes |
| DD | Define Doubleword | allocates 4 bytes |
| DQ | Define Quadword | allocates 8 bytes |
| DT | Define Ten Bytes | allocates 10 byte |
+-----------+-------------------+-------------------+For example:
choice DB 'y'
number DW 12345Display the value of ecx register which is a char
pointer (in other words, print the string referred to):
display (char *) $ecxNote, this will display the value at every break of the program execution, including each step if you are stepping through. To stop this behavior:
undisplay 1Note that the number can be two, three, or something else if there are multiple variables in display mode.
NASM
GDB
ASSEMBLY
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NASM
LD Linker
Assembly
Call Stack
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