Standard Calling Convention
January 2, 2025 ยท View on GitHub
This is the standard calling convention for the RC800 family. Other calling conventions are possible, this is the officially suggested one.
Notation
As each machine register is a stack, and parameters may be passed on these stacks, stack values are represented by adding apostrophes to the register mnemonic.
FT refers to the current (topmost) value of the FT register.
FT' is the next value on the stack, the one that will appear after a POP FT instruction.
FT'' is the next value again.
and so forth.
R may refer to any 16 bit register.
multi-word refers to a value or object larger than 16 bits.
Basic passing of parameters to subroutines
Eight bit parameters can be passed in the first available 8 bit register, starting from T and then B, C, D and E. HL is not used as this is will hold the return address. F is not used to pass an 8 bit value.
Sixteen bit parameters can be passed in the first available 16 bit register, starting from FT and then BC and DE. HL is not used as this will hold the return address.
Returning values
Values are returned in the FT register or, in the case of multi-word values, on the FT stack.
Passing multi-word values
Parameters 32 bits or larger can be passed on one register stack. In the case of 32 bit integers, R and R' hold the two 16 bit words, the most significant word in R.
Preserved registers
The convention is generally "callee-saves". All values, including the ones held on the stack, must be preserved by the callee for registers BC, DE and HL.
The register FT is "caller-saves". FT must be assumed to be destroyed by the callee, even if the callee does not return a value.
If a multi-word parameter is passed on the FT stack, the callee will consume this value and remove it from the stack. Note that "consuming" a value means removing one less word than the value actually uses. In the case of a 32 bit value, it takes up one extra word of stack space, thus only one word should be removed by the callee.