arm64
This page only talks about the 64 bit part of the
armv8
architecture. For an overview seearmv8
.
keywords: arm64, aarch64, abi
- 64bit synonyms:
arm64
,aarch64
- ISA type:
RISC
- Endianness:
little
,big
Registers
General purpose registers
bytes
[7:0] [3:0] desc
---------------------------------------------
x0-x28 w0-w28 general purpose registers
x29 w29 frame pointer (FP)
x30 w30 link register (LR)
sp wsp stack pointer (SP)
pc program counter (PC)
xzr wzr zero register
Write to
wN
register clears upper 32bit.
Special registers per EL
bytes
[7:0] desc
---------------------------------------------
sp_el0 stack pointer EL0
sp_el1 stack pointer EL1
elr_el1 exception link register EL1
spsr_el1 saved process status register EL1
sp_el2 stack pointer EL2
elr_el2 exception link register EL2
spsr_el2 saved process status register EL2
sp_el3 stack pointer EL3
elr_el3 exception link register EL3
spsr_el3 saved process status register EL3
Instructions cheatsheet
Accessing system registers
Reading from system registers:
mrs x0, vbar_el1 // move vbar_el1 into x0
Writing to system registers:
msr vbar_el1, x0 // move x0 into vbar_el1
Control Flow
b <offset> // relative forward/back branch
br <Xn> // absolute branch to address in register Xn
// branch & link, store return address in X30 (LR)
bl <offset> // relative forward/back branch
blr <Xn> // absolute branch to address in register Xn
ret {Xn} // return to address in X30, or Xn if supplied
Addressing
Offset
ldr x0, [x1] // x0 = [x1]
ldr x0, [x1, 8] // x0 = [x1 + 8]
ldr x0, [x1, x2, lsl #3] // x0 = [x1 + (x2<<3)]
ldr x0, [x1, w2, stxw] // x0 = [x1 + sign_ext(w2)]
ldr x0, [x1, w2, stxw #3] // x0 = [x1 + (sign_ext(w2)<<3)]
Shift amount can either be
0
orlog2(access_size_bytes)
. Eg for 8byte access it can either be{0, 3}
.
Index
ldr x0, [x1, 8]! // pre-inc : x1+=8; x0 = [x1]
ldr x0, [x1], 8 // post-inc: x0 = [x1]; x1+=8
Pair access
ldp x1, x2, [x0] // x1 = [x0]; x2 = [x0 + 8]
stp x1, x2, [x0] // [x0] = x1; [x0 + 8] = x2
Procedure Call Standard ARM64 (aapcs64
)
Passing arguments to functions
- Integer/Pointer arguments
reg arg ----------- x0 1 .. .. x7 8
- Additional arguments are passed on the stack. Arguments are pushed
right-to-left (RTL)
, meaning next arguments are closer to currentsp
.void take(..., int a9, int a10); | | | ... | Hi | +-->| a10 | | +---------->| a9 | <-SP | +-----+ v | ... | Lo
Return values from functions
- Integer/Pointer return values
reg size ----------------- x0 64 bit
Callee saved registers
x19
-x28
SP
Stack
- full descending
- full:
sp
points to the last used location (valid item) - descending: stack grows downwards
- full:
sp
must be 16byte aligned when used to access memory for r/wsp
must be 16byte aligned on public interface interfaces
Frame chain
- linked list of stack-frames
- each frame links to the frame of its caller by a
frame record
- a frame record is described as a
(FP,LR)
pair
- a frame record is described as a
x29 (FP)
must point to the frame record of the current stack-frame+------+ Hi | 0 | frame0 | +->| 0 | | | | ... | | | +------+ | | | LR | frame1 | +--| FP |<-+ | | ... | | | +------+ | | | LR | | current | x29 ->| FP |--+ frame v | ... | Lo
- end of the frame chain is indicated by following frame record
(0,-)
- location of the frame record in the stack frame is not specified
Function prologue & epilogue
- prologue
sub sp, sp, 16 stp x29, x30, [sp] // [sp] = x29; [sp + 8] = x30 mov x29, sp // FP points to frame record
- epilogue
ldp x29, x30, [sp] // x29 = [sp]; x30 = [sp + 8] add sp, sp, 16 ret
ASM skeleton
Small assembler skeleton, ready to use with following properties:
- use raw Linux syscalls (
man 2 syscall
for ABI) - no
C runtime (crt)
- gnu assembler
gas
// file: greet.S
#include <asm/unistd.h> // syscall NRs
.arch armv8-a
.section .text, "ax", @progbits
.balign 4 // align code on 4byte boundary
.global _start
_start:
mov x0, 2 // fd
ldr x1, =greeting // buf
ldr x2, =greeting_len // &len
ldr x2, [x2] // len
mov w8, __NR_write // write(2) syscall
svc 0
mov x0, 0 // exit code
mov w8, __NR_exit // exit(2) syscall
svc 0
.balign 8 // align data on 8byte boundary
.section .rodata, "a", @progbits
greeting:
.asciz "Hi ASM-World!\n"
greeting_len:
.int .-greeting
man gcc:
file.S
assembler code that must be preprocessed.
To cross-compile and run:
> aarch64-linux-gnu-g++ -o greet greet.S -nostartfiles -nostdlib \
-Wl,--dynamic-linker=/usr/aarch64-linux-gnu/lib/ld-linux-aarch64.so.1 \
&& qemu-aarch64 ./greet
Hi ASM-World!
Cross-compiling on
Ubuntu 20.04 (x86_64)
, paths might differ on other distributions. Explicitly specifying the dynamic linker should not be required when compiling natively on arm64.