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

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[{TableOfContents }] \\
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!! OVERVIEW
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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.
This page (revision-7) was last changed on 03-Feb-2023 15:21 by Carsten Strotmann  Top