A FORTH ASSEMBLER FOR THE 6502#

by William F. Ragdale

Source: 6502 Assembler in Forth

INTRODUCTION#

This article should further polarize the attitudes of those outside the growing community of FORTH users. Some will be fascinated by a label-less, macro-assembler whose source code is only 96 lines. Others will be repelled by reverse Polish syntax and the absence of labels.

The author immodestly claim that this is the best FORTH assembler ever distributed. It is the only such assembler that detects all errors in op-code generation and conditional structuring. It is released to the public domain as a defense mechanism. Three good 6502 assemblers were submitted to the FORTH Interest Group but each had some lack. Rather than merge and edit for publication, I chose to publish mine with all the submitted features plus several more.

Imagine having an assembler in 1300 bytes of object code with:

1. User macros (like IF,UNTIL,) definable at any time.
2. Literal values expressed in any numeric base, alterable at any time.
3. Expressions using any resident computation capability.
4. Nested control structures without labels, with error control.
5. Assembler source itself in a portable high level language.

OVERVIEW#

Forth is provided with a machine language assembler to create execution procedures that would be time inefficient, if written as colon-definitions. It is intended that "code" be written similarly to high level, for clarity of expression. Functions may be written first in high-level, tested, and then re-coded into assembly, with a minimum of restructuring.

THE ASSEMBLY PROCESS#

Code assembly just consists of interpreting with the ASSEMBLER vocabulary as CONTEXT. Thus, each word in the input stream will be matched according the Forth practice of searching CONTEXT first then CURRENT:

ASSEMBER   (now CONTEXT)
FORTH      (chained to ASSEMBLER)
user's     (CURRENT if one exists)
FORTH      (chained to the user's vocab)
try for literal number
else, do error abort

The above sequence is the usual action of Forth's text interpreter, which remains in control during assembly.

During assembly of CODE definitions, Forth continues interpretation of each word encountered in the input stream (not in the compile mode). These assembler words specify operands, address modes, and opcodes. At the conclusion of the CODE definition a final error check verifies correct completion by "unsmudging" the definition's name, to make it available for dictionary searches.

RUN-TIME, ASSEMBLY-TIME#

One must be careful to understand at what time a particular word definition executes. During assembly, each assembler word interpreted executes. Its function at that instant is called 'assembling' or 'assembly-time'. This function may involve op-code generation, address calculation, mode selection, etc.

The later execution of the generated code is called 'run-time'. This distinction is particularly important with the conditionals. At assembly time each such word (i.e., IF,UNTIL, BEGIN, etc.) itself 'runs' to produce machine code which will later execute at what is labeled 'run-time' when its named code definition is used.

(work in Progress)