# arm64
keywords: arm64, aarch64, abi
- 64bit synonyms: `arm64`, `aarch64`
- ISA type: `RISC`
- Endianness: `little`, `big`
## Registers
### General purpose registers
```markdown
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
```markdown
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:
```armasm
mrs x0, vbar_el1 // move vbar_el1 into x0
```
Writing to system registers:
```armasm
msr vbar_el1, x0 // move x0 into vbar_el1
```
### Control Flow
```armasm
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
```armasm
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` or `log2(access_size_bytes)`. Eg for 8byte
> access it can either be `{0, 3}`.
### Index
```armasm
ldr x0, [x1, 8]! // pre-inc : x1+=8; x0 = [x1]
ldr x0, [x1], 8 // post-inc: x0 = [x1]; x1+=8
```
### Pair access
```armasm
ldp x1, x2, [x0] // x1 = [x0]; x2 = [x0 + 8]
stp x1, x2, [x0] // [x0] = x1; [x0 + 8] = x2
```
## Procedure Call Standard ARM64 ([`aapcs64`][aapcs64])
### Passing arguments to functions
- Integer/Pointer arguments
```markdown
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 current `sp`.
```markdown
void take(..., int a9, int a10);
| | | ... | Hi
| +-->| a10 | |
+---------->| a9 | <-SP |
+-----+ v
| ... | Lo
```
### Return values from functions
- Integer/Pointer return values
```markdown
reg size
-----------------
x0 64 bit
```
### Callee saved registers
- `x19` - `x28`
- `SP`
### Stack
- grows downwards
- `sp` must be 16byte aligned when used to access memory for r/w
- `sp` 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
- `x29 (FP)` must point to the frame record of the current stack-frame
```markdown
+------+ 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
```armasm
sub sp, sp, 16
stp x29, x30, [sp] // [sp] = x29; [sp + 8] = x30
mov x29, sp // FP points to frame record
```
- epilogue
```armasm
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`][gas_doc]
```armasm
// 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:
```bash
> 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.
## References
- [Procedure Call Standard ARM64][aapcs64]
- [ARMv8-A Programmer's Guide][armv8a_prog_guide]
- [ARMv8-A Architecture Reference Manual][armv8a_arm]
- [AppNote: ARMv8 Bare-metal boot code][armv8_baremetal_boot]
- [GNU Assembler][gas_doc]
- [GNU Assembler Directives][gas_directives]
[aapcs64]: https://github.com/ARM-software/abi-aa/blob/master/aapcs64/aapcs64.rst
[armv8a_prog_guide]: https://developer.arm.com/documentation/den0024/a
[armv8a_arm]: https://developer.arm.com/documentation/ddi0487/latest
[armv8_baremetal_boot]: https://developer.arm.com/documentation/dai0527/a
[gas_doc]: https://sourceware.org/binutils/docs/as
[gas_directives]: https://sourceware.org/binutils/docs/as/Pseudo-Ops.html#Pseudo-Ops
