ASM

PET VIC-20 C64 C16 Plus/4 C128

Syntax

ASM 
  <assembly code>
  <assembly code>
END ASM

The ASM directive injects bare 6502 assembly code into the compiled XC=BASIC code without any modification. As XC=BASIC itself produces dasm code, the injected assembly code must also be in dasm format and syntax.

Example

REM -- set border to black using XC=BASIC code
POKE $d020, 0
REM -- set background to black using inline assembly
ASM
 lda #$00
 sta $d021
END ASM
REM -- the XC=BASIC program continues on from here
PRINT "done"
END

The above example illustrates how to use ASM to inject inline assembly code into the XC=BASIC program. The compiler will send the code between the ASM … END ASM commands straight to dasm for assembly immediately following the POKE command. Newlines are used in this example to improve the readability of 6502 assembly code that spans beyond one line. Note there are no quotation marks or other encapsulations around the assembly code as was previously required in XC=BASIC v2.

Note the spaces before the opcodes on each line of dasm assembly code. Leading spaces are required by dasm for any line of assembly that is not a valid label in dasm.

Warning

Each line in dasm assembly code must start with a label or at least one space.

Note

The XC=BASIC compiler cannot validate the in-line assembly code. It will be copied verbatim into the XC=BASIC assembly before being compiled by dasm, so you must take special care with the ASM directive. Dasm will report any errors. Please consult the dasm docs carefully for syntax and formatting.

Referencing BASIC Non-String Variables in Assembly Code

To use BASIC non-string variables in assembly code, use the syntax {variable_name}. For example:

a = 2 : REM global variable
b = 7
SUB test () STATIC
  DIM a AS BYTE : REM local variable
  a = 5
  ASM
    lda #$06
    sta {a} ; {a} is resolved to the local "a"
    sta {b}
  END ASM
  PRINT a
END SUB

CALL test ()
PRINT a
PRINT b

Warning

Only STATIC variables can be referenced using the curly braces syntax.

Referencing Strings in Assembly Code

Passing an XC=BASIC string by reference will not always produce expected results, because XC=BASIC strings in memory are preceded by a single byte string length and are not zero-terminated. Therefore, strings for operations like KERNAL calls (SETNAM, for example) must be be converted first. There are a couple of techniques you can use to accomplish this:

  • Place the string into unused memory and terminate it with a zero byte, then reference that location in the assembly code
  • Create a byte array with stringlen + 1 elements, convert the string to a KERNAL-friendly format, then pass the array by reference in the assembly code

An example of the byte array technique:

' create a byte array, fill it with the string "file," and zero-terminate it
DIM fname(5) AS BYTE
fname(0) = ASC("f")
fname(1) = ASC("i")
fname(2) = ASC("l")
fname(3) = ASC("e")
fname(4) = 0

' set KERNAL SETNAM routine to point to fname
ASM
  lda #$01
  ldx #<{fname}    ; set lo byte to fname lo
  ldy #>{fname}    ; set hi byte to fname hi 
  jsr $ffbd    ; SETNAM Kernal routine
END ASM

Or, if you have a longer string that you want to pass, consider iterating over the string with MID$ (see the following example) to simplify the conversion of a string into a byte array. Note that XC=BASIC will reset the string variable's length to the actual length of the assigned string every time a string is assigned to that variable, so you don't need to worry about calculating the length using some other trick if you've assigned a string shorter than the full dimension of the variable. LEN(string) will work:

DIM a(22) AS BYTE
DIM x AS BYTE
DIM b AS STRING * 20
b = "the quick brown fox"

' will result in a byte array of "the quick brown fox"
' and a zero byte in the next array element, ready
' for use by a KERNAL call
FOR x = 0 to LEN(b)
    a(x) = ASC(MID$(b, x, 1))
NEXT
a(len(b) + 1) = 0