AArch64
ARM's 64-bit Architecture
by David Thomas — JANUARY 2014
(UNFINISHED)
Expected Audience
Engineers familiar with ARM
Introduction
ARM's new 64-bit architecture
(It's not that new: its existence has been public since ~2011)
Already available in Apple hardware
iPhone 5, iPad Mini Retina, iPad Air
Soon to be available in Android hardware
The 64-bit ARM architecture is called AArch64 not ARM64
(Apple's tools go and call it 'arm64' anyway.)
The 32-bit ARM+Thumb architecture is retconned AArch32
AArch32 + AArch64 = ARMv8
Register Set
- 32 x 64-bit wide registers
- Generally accessible as x0..x30
- Special registers
- x30 - link register
- x31 - multipurpose register; sometimes reads as zero
- Registers are addressable as both 32-bit and 64-bit
- 32-bit ops use the bottom half of the 64-bit registers
- There are widening and sign extending load ops etc.
Instruction Set
Substantial changes from ARM
- Conditional execution is gone :-[
- But it's for the best: branch predictors have obsoleted it
- No multi-reg load/store ops
- But you do get paired load/stores
- New fused test-and-branch ops
- Divide instructions
- No equivalent of Thumb
Procedure Calling StandarD
ARM define the AArch64 PCS, [link].
Of course, Apple have to have their own variant on it, [link].
Common Patterns
Forming Large Constants
-
(Like ARM) form constants by moving pieces into a register
- e.g. To move 0xA1B2C3D4 into xN
movz xN, #0xA1B2, lsl #16 ; move and zero the other register bits (xN = 0x00000000A1B20000)
movk xN, #0xC3D4 ; move and keep the other register bits (xN = 0x00000000A1B2C3D4)
-
(Like ARM) only worthwhile for certain constants
- Load from memory for complex constants
-
(Like ARM) can invert the sense of loads for more range
movn xN, #0xA1B2, lsl #16 ; move NOT (xN = 0xFFFFFFFF5E4DFFFF)
Common Patterns
Function entry & exit
Makes use of the load-store pair instructions
stp x29, x30, [sp, #-16]!
mov x29, sp
<function body>
ldp x29, x30, [sp], #16
Register x29 here is the frame pointer
COMMON PATTERNS
Load 32-bit valueS as 64-bit
ldrsw x2, [x8] ; load register x2 with the Signed Word at x8
COMMON PATTERNS
Sequence of compares
Sequences of conditional comparisons can be chained
cmp x8, #0x0 ; flags = (x8 cmp 0x0)
ccmp x1, #0x0, #0x4, ne ; if (ne) flags = (x1 cmp 0x0) else flags = 0x4;
ccmp x20, #0x0, #0x4, ne ; if (ne) flags = (x20 cmp 0x0) else flags = 0x4;
b.eq 0x1879c ; if (eq) branch ...
Somewhat baroque...
Alternative Syntaxes
The implict xzr zero register can
appear in some instructions' disassembly:
orr x9, xzr, #0x01
Were we writing this by hand we'd use:
mov x9, #0x01
Ambiguous: could also be assembled as:
movz x9, #0x01
Aliases:
...ought to be disassembled as