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+# 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