Calling conventions: make functions agree

For ordinary integer or pointer arguments in the System V AMD64 register classes, the first six argument registers are RDI, RSI, RDX, RCX, R8, and R9. A simple integer return value commonly uses RAX. Classification rules become more involved for aggregates, floating-point values, variadic functions, and wide vectors, so this six-register list is a deliberately bounded starting point. A compiler knows those classifications and emits code that lets independently compiled caller and callee agree without sharing local variable names.

Caller-saved registers may be changed by the callee; a caller that needs their values afterward must arrange preservation. Callee-saved RBX, RBP, and R12 through R15 must be restored if used, and RSP must be correctly restored for returning. This division is an ownership rule rather than a promise that every function saves every register. A leaf function that changes none of the preserved registers can avoid those saves. Inspect actual liveness before attributing every register move to the ABI.

For an ordinary call with the base 16-byte stack alignment requirement, the caller arranges RSP to be divisible by 16 immediately before CALL. CALL pushes an eight-byte return address, so the callee’s entry RSP is eight modulo 16. Equivalently, RSP plus eight is aligned at entry. Saving one eight-byte register changes the remainder again. Stronger alignment requirements apply for some stack-passed vector arguments, which this example excludes. Work in bytes and check the boundary at which the ABI states its rule.

A stack frame is storage associated with an active call, not necessarily a fixed picture with every field present. A compiler may omit a dedicated frame pointer, keep locals in registers, inline a call, or perform a tail call that reuses the caller’s return path. Debugging and unwinding can use metadata rather than a linked list of RBP frames. The user-space red zone also has specific ABI conditions; do not copy assumptions about it into kernel code or a different calling convention.

Handwritten assembly, foreign-function interfaces, and JIT-generated code must obey the same visible calling contract as compiler-generated functions. A function that produces the expected result but corrupts RBX can fail much later when its caller reuses that register. Likewise, a stack-alignment violation can remain hidden until a callee performs an operation with stricter assumptions. Test boundaries with representative signatures and preserved-register checks, not only a trivial call that happens to return the right integer.

The network lab exposes a single sigmoid computation with input x and weight w. It is useful as an analogy for an interface: predict the output from the documented inputs, then change one input while holding the other fixed. The diagram does not represent argument registers, a call stack, or a trainable network. The implementation-independent idea is that a caller can reason from a stable contract. Use a separate register-and-stack trace when applying that idea to an actual binary interface.