Article by Ayman Alheraki on January 11 2026 10:37 AM
which is utilized by processors like the Snapdragon X Elite. Below is a comprehensive overview of:
General-Purpose Registers
Condition Flags (NZCV)
Instruction Encoding with Hexadecimal Examples
AI/ML Instruction Extensions (NEON, SVE)
ARMv8-A defines 31 general-purpose 64-bit registers, labeled X0 through X30. Each can also be accessed as 32-bit W0–W30. Register X31 serves as the zero register (when used as a source) or the stack pointer (SP) (when used as a destination).Wikipedia+7Stack Overflow+7cs140e.sergio.bz+7
| Register | 64-bit Name | 32-bit Name | Description |
|---|---|---|---|
| X0–X7 | X0–X7 | W0–W7 | Function arguments / return values |
| X8 | X8 | W8 | Indirect result location register |
| X9–X15 | X9–X15 | W9–W15 | Temporary registers |
| X16–X17 | X16–X17 | W16–W17 | Intra-procedure-call scratch registers |
| X18 | X18 | W18 | Platform register (reserved) |
| X19–X28 | X19–X28 | W19–W28 | Callee-saved registers |
| X29 | FP | W29 | Frame pointer |
| X30 | LR | W30 | Link register (return address) |
| X31 | SP or ZR | WSP or WZR | Stack pointer or zero register |
The PSTATE register holds condition flags that reflect the outcomes of operations. These flags are:Arm Community+2Stack Overflow+2Arm Documentation Service+2
| Flag | Bit Position | Description |
|---|---|---|
| N | 31 | Negative: Set if the result is negative |
| Z | 30 | Zero: Set if the result is zero |
| C | 29 | Carry: Set if there's a carry out |
| V | 28 | Overflow: Set if there's an overflow |
These flags are updated by instructions that have an 'S' suffix, such as ADDS or SUBS.arm-condition-flags.vercel.app
ARMv8-A instructions are uniformly 32 bits (4 bytes) in length and are encoded in little-endian format. Below are examples of common instructions with their hexadecimal representations:Stack Overflow
| Instruction | Description | Hex Encoding |
|---|---|---|
MOV X0, #1 | Move immediate value 1 into X0 | 0xD2800020 |
ADD X1, X2, X3 | Add X2 and X3, store result in X1 | 0x8B030041 |
SUB X4, X5, #10 | Subtract immediate 10 from X5, store in X4 | 0xD1001454 |
| Instruction | Description | Hex Encoding |
|---|---|---|
AND X6, X7, X8 | Bitwise AND of X7 and X8, result in X6 | 0x8A0800E6 |
ORR X9, X10, X11 | Bitwise OR of X10 and X11, result in X9 | 0xAA0B0149 |
EOR X12, X13, X14 | Bitwise XOR of X13 and X14, result in X12 | 0xCA0E01AC |
| Instruction | Description | Hex Encoding |
|---|---|---|
B label | Unconditional branch to label | 0x14000000 |
BL label | Branch with link (call function) | 0x94000000 |
CBZ X0, label | Compare X0 with zero, branch if zero | 0xB4000000 |
Note: The actual hexadecimal encoding for branch instructions depends on the offset to the label.
ARMv8-A introduces extensions like NEON and SVE (Scalable Vector Extension) to accelerate AI and machine learning workloads.
NEON provides 128-bit vector operations. Examples include:
| Instruction | Description | Hex Encoding |
|---|---|---|
ADD V0.4S, V1.4S, V2.4S | Add four 32-bit integers from V1 and V2, store in V0 | 0x4E208020 |
MUL V3.4S, V4.4S, V5.4S | Multiply four 32-bit integers from V4 and V5, store in V3 | 0x6E205883 |
SVE allows for vector lengths beyond 128 bits, adapting to the hardware's capabilities. Instruction encodings for SVE are more complex and depend on the specific vector length implemented in the hardware.
For detailed information on ARM instruction encodings and extensions:
ARM A64 Instruction Set Architecture
Condition Codes and Flags in ARM