Syscall ABI

To write

The register-level contract, per architecture. Facts only; the C view is in Syscalls.

ARMv7-A

• syscall number in r12; svc #0x112023 (ARM) or svc #0x23 (Thumb), the 2023-11-20 magic
• arguments in r0–r3, results in r0–r3, r0 is the status (negative = error)
• registers preserved across a call

Stability

• numbers never reused within a major version; what changed from 1.x