c64asm ASM syntax

mikefc

2023-09-29

Syntax overview

The basic syntax of 6502 assemblers are mostly the same. For c64asm the basic syntax is modelled on TASS64.

Extensions to this basic syntax have been made to include some R specific features for easier passing of data to the assembler

Values - hexademical only

• hexadecimal bytes must be 1 or 2 characters, and must be preceded by $ e.g.
  • allowed: $01, $f0
  • not allowed: $1, $f00, ff
• two-byte hexadecimal values must be 3 or 4 characters, and must be preceded by $ e.g.
  • allowed: $0001, $d0f0
  • not allowed: $1, d0f0

Variables (include the program counter *)

• variables should be alphanumeric and not contain spaces
• values assigned with =, e.g.
  • * = $0820
  • border = $d020

Address labels

• address labels should be alphanumeric and not contain spaces

R-specific: .rtext

• if the variable mystring is a character string in the R session, then this may be included into the assembly code using:
  • .rtext mystring

R-specific: .byte

• if the variable mybytes is an integer vector in the R session, then this may be included into the assembly code using:
  • .rbyte mbytes
• The vector must be integer
• The values must be in the range 0-255

R-specific: {...} for evaluated expressions

• Expressions with {...} are evaluated at compile time.
  
• These can contain any R expression, but would most often be used to calculate offsets from known address labels, e.g.
  • if border = $d020, then sta {border + 1} is equivalent to sta $d021
• In other assemblers, * is used to refer to the program counter/current address in this type of operation. However, in order to avoid clashes with the multiplication operator, within an evaluated expression use .* instead. A bare * will be treated as a normal multiply.