millfork/docs/abi/calling-convention.md

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Millfork calling convention

Note: all the info below may change without any warning and is given only for debugging purposes.

6502

Parameters:

• if the function has one parameter of size one byte, it is passed via the A register

• otherwise, all parameters are passed via static locations

Return values:

• one-byte return values are passed via the A register

• two-byte return values are passed via the A (low byte) and X (high byte) register

Register preservation:

• callee may clobber all three registers (A, X, Y) and most flags (Z, V, C, N, and also I if using inline assembly)

• callee expects the D flag to be clear and will leave it clear

• on 65816: callee will preserve the emulation flag (setting the emulation flag correctly is the responsibility of the initialization code)

• on 65816 in native mode:

  • callee expects the M and X flag to be set and will leave them set (8-bit accumulator and index registers by default)

  • callee expects the direct page register to be set to 0000 and will not change it

Z80

Parameters:

• if the function has one parameter of size one byte, it is passed via the A register

• if the function has one parameter of size two bytes, it is passed via the HL register pair

• otherwise, all parameters are passed via static locations

Return values:

• one-byte return values are passed via the A register

• two-byte return values are passed via the HL register pair

Register preservation:

• callee may clobber all flags

• callee may clobber all registers except for IX, IY and shadow registers