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A FORTH ASSEMBLER FOR THE 6502
A FORTH ASSEMBLER FOR THE 6502

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!! AN EXAMPLE
As a practical example, here's a simple call to the system monitor, via the NMI address vector (using the BRK opcode).
{{{
CODE MON ( exit to monitor )
BRK,
NEXT JMP,
END-CODE
}}}
The word CODE is first encountered, and executed by Forth. CODE builds the following name "MON" into a dictionary header and calls ASSEMBLER as the CONTEXT vocabulary.
The "(" is next found in FORTH and executed to skip til ")". This method skips over comments. Note that the name after CODE and the ")" after "(" must be on the same text line.
!! OP-CODES
BRK, is next found. in the assembler as the op-code. When BRK, executes, it assembles the byte value 00 into the dictionary as the op-code for "break to monitor via "NMI".
Many assembler word names end in ",". The significance of this is:
# The comma shows the conclusion of a logical grouping that would be one line of classical assembly source code.
# "," compiles into the dictionary, thus a comma implies the point at which code is generated.
# The "," distinguishes op-codes from possible hex numbers ADC and ADD.
!! NEXT
Forth executes your word definition under control of the address interpreter, named NEXT. This short code routine moves execution from one definition, to the next. At the end of your code definition, you must return control to NEXT or else to code which returns to NEXT.
!! RETURN OF CONTROL
Most 6502 systems can resume execution after a break, since the monitor saves the CPU register contents. Therefore, we must return control to Forth after a return from the monitor. NEXT is a constant that specifies the machine address of Forth's address interpreter (say $0242). Here it is the operand for "JMP,". As "JMP," executes, it assembles a machine code jump to the address of NEXT from the assembly time stack value.
This page (revision-7) was last changed on 03-Feb-2023 15:21 by Carsten Strotmann  Top