# armv7a
keywords: arm, armv7, abi
- ISA type: `RISC`
- Endianness: `little`, `big`
## Registers
### General purpose registers
```markdown
bytes
[3:0] alt desc
---------------------------------------------
r0-r12 general purpose registers
r11 fp
r13 sp stack pointer
r14 lr link register
r15 pc program counter
```
### Special registers
```markdown
bytes
[3:0] desc
---------------------------------------------
cpsr current program status register
```
### CPSR register
```markdown
cpsr
bits desc
-----------------------------
[31] N negative flag
[30] Z zero flag
[29] C carry flag
[28] V overflow flag
[27] Q cummulative saturation (sticky)
[9] E load/store endianness
[8] A disable asynchronous aborts
[7] I disable IRQ
[6] F disable FIQ
[5] T indicate Thumb state
[4:0] M process mode (USR, FIQ, IRQ, SVC, ABT, UND, SYS)
```
## Instructions cheatsheet
### Accessing system registers
Reading from system registers:
```armasm
mrs r0, cpsr // move cpsr into r0
```
Writing to system registers:
```armasm
msr cpsr, r0 // move r0 into cpsr
```
### Control Flow
```armasm
b <lable> // relative forward/back branch
bl <lable> // relative forward/back branch & link return addr in r14 (LR)
// branch & exchange (can change between ARM & Thumb instruction set)
// bit Rm[0] == 0 -> ARM
// bit Rm[0] == 1 -> Thumb
bx <Rm> // absolute branch to address in register Rm
blx <Rm> // absolute branch to address in register Rm &
// link return addr in r14 (LR)
```
## Procedure Call Standard ARM ([`aapcs32`][aapcs32])
### Passing arguments to functions
- integer/pointer arguments
```markdown
reg arg
-----------
r0 1
.. ..
r3 4
```
- a double word (64bit) is passed in two consecutive registers (eg `r1+r2`)
- 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 a5, int a6);
| | | ... | Hi
| +-->| a6 | |
+---------->| a5 | <-SP |
+-----+ v
| ... | Lo
```
### Return values from functions
- integer/pointer return values
```markdown
reg size
-----------------
r0 32 bit
r0+r1 64 bit
```
### Callee saved registers
- `r4` - `r11`
- `sp`
### Stack
- full descending
- full: `sp` points to the last used location (valid item)
- descending: stack grows downwards
- `sp` must be 4byte aligned (word boundary) at all time
- `sp` must be 8byte aligned on public interface interfaces
### Frame chain
- not strictly required by each platform
- 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 (2x32bit)
- `r11 (FP)` must point to the frame record of the current stack-frame
```markdown
+------+ Hi
| 0 | frame0 |
+->| 0 | |
| | ... | |
| +------+ |
| | LR | frame1 |
+--| FP |<-+ |
| ... | | |
+------+ | |
| LR | | current |
r11 ->| 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
- `r11` is not updated before the new frame record is fully constructed
### Function prologue & epilogue
- prologue
```armasm
push {fp, lr}
mov fp, sp // FP points to frame record
```
- epilogue
```armasm
pop {fp, pc} // pop LR directly into PC
```
## 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 armv7-a
.section .text, "ax"
.balign 4
// Emit `arm` instructions, same as `.arm` directive.
.code 32
.global _start
_start:
// Branch with link and exchange instruction set.
blx _do_greet
mov r0, #0 // exit code
mov r7, #__NR_exit // exit(2) syscall
swi 0x0
// Emit `thumb` instructions, same as `.thumb` directive.
.code 16
.thumb_func
_do_greet:
mov r0, #2 // fd
ldr r1, =greeting // buf
ldr r2, =greeting_len // &len
ldr r2, [r2] // len
mov r7, #__NR_write // write(2) syscall
swi 0x0
// Branch and exchange instruction set.
bx lr
.balign 8 // align data on 8byte boundary
.section .rodata, "a"
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
> arm-linux-gnueabi-gcc -o greet greet.S -nostartfiles -nostdlib \
-Wl,--dynamic-linker=/usr/arm-linux-gnueabi/lib/ld-linux.so.3 \
&& qemu-arm ./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 arm.
## References
- [Procedure Call Standard ARM][aapcs32]
- [ARMv7-A Programmer's Guide][armv7a_prog_guide]
- [ARMv7-A Architecture Reference Manual][armv7a_arm]
- [GNU Assembler][gas_doc]
- [GNU Assembler Directives][gas_directives]
- [GNU Assembler `ARM` dependent features][gas_arm]
[aapcs32]: https://github.com/ARM-software/abi-aa/blob/master/aapcs32/aapcs32.rst
[armv7a_prog_guide]: https://developer.arm.com/documentation/den0013/latest
[armv7a_arm]: https://developer.arm.com/documentation/ddi0406/latest
[gas_doc]: https://sourceware.org/binutils/docs/as
[gas_directives]: https://sourceware.org/binutils/docs/as/Pseudo-Ops.html#Pseudo-Ops
[gas_arm]: https://sourceware.org/binutils/docs/as/ARM_002dDependent.html
