From 871f1a6f408a9b50182c8688d4aeec26bd6b2d93 Mon Sep 17 00:00:00 2001
From: johannst <johannes.stoelp@gmail.com>
Date: Tue, 29 Sep 2020 00:58:29 +0200
Subject: arm64: added basic arch/pcs

---
 src/arch/arm64.md | 137 +++++++++++++++++++++++++++++++++++++++++++++++++++++-
 1 file changed, 135 insertions(+), 2 deletions(-)

(limited to 'src/arch')

diff --git a/src/arch/arm64.md b/src/arch/arm64.md
index 2286bd8..605697f 100644
--- a/src/arch/arm64.md
+++ b/src/arch/arm64.md
@@ -5,6 +5,139 @@ keywords: arm64, aarch64, abi
 - 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
+```
+
+## 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:
@@ -19,7 +152,7 @@ Small assembler skeleton, ready to use with following properties:
     .arch armv8-a
 
     .section .text, "ax", @progbits
-    .align 2
+    .balign 4                // align code on 4byte boundary
     .global _start
 _start:
     mov x0, 2                // fd
@@ -33,7 +166,7 @@ _start:
     mov w8, __NR_exit        // exit(2) syscall
     svc 0
 
-    .align 3
+    .balign 8                // align data on 8byte boundary
     .section .rodata, "a", @progbits
 greeting:
     .asciz "Hi ASM-World!\n"
-- 
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