Skip to content

FASTCALL

FastCall is used to indicate that a SUB or FUNCTION will follow the FastCall Calling Convention. In this convention, the 1st parameter of the function will come in the registers as follows:

  • If the function takes a Byte (or UByte) parameter, then the A register will be set with this parameter.
  • If it takes an Integer (or UInteger) parameter, then the HL register will be set with the value of that parameter on entry to the function.
  • If it takes a Long (or ULong), or fixed parameter, then the DE and HL registers will hold the 32bit value of the parameter, where HL holds the lower 16 bits and DE the higher one.
  • If it takes a Float type parameter, then the registers C, DE and HL will hold the five bytes for that value. Here, C is the exponent (excess 127), and DEHL the mantissa, being DE the highest 16 bits and HL the lower ones.

Return is automatic based on the function return type in the same way: * 8bit returns should be in the A register * 16bit returns should be in HL * 32bit returns should be in DEHL * 40bit FLOAT returns should be in CDEHL.

Strings are a 16bit UInteger pointer, hence the HL register will be used (both for parameters and return values)

FastCall should ONLY be used with functions that take a single parameter. If you use more than one parameter, the remaining parameters will come in the stack. Upon return, you will have to deal with the stack (SP register) and restore it to the state it had prior your function was called.

When entering the function, the stack will be as follows:

  • [SP + 00]: Return address. Top of the stack (16 bits)
  • [SP + 02]: 2nd parameter (if any)

Example

 1
 2
 3
 4
 5
 6
 7
 8
 9
10
11
12
13
14
FUNCTION FASTCALL whatLetter (A as uByte) as uByte
  Asm
            PROC
            LOCAL DATA, START
            JP START
  DATA:     DEFB "A Man, A Plan, A Canal, Panama"
  START:    LD HL,DATA
            LD D,0
            LD E,A
            ADD HL,DE
            LD A,(HL)
            ENDP
  End Asm
END FUNCTION

The above function, when called with whatLetter(<value>) will return the <value>-th letter of the phrase "A Man, A Plan, A Canal, Panama".

Notes

  • Note that the A register already contains <value> when the inline assembly is reached.
  • Note that we do NOT need to put a ret opcode on the end of the assembly. The compiler will do that for us.

See Also