This page (revision-7) was last changed on 03-Feb-2023 15:21 by Carsten Strotmann 

This page was created on 25-Apr-2010 09:19 by Carsten Strotmann

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

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At line 80 added one line
!! SECURITY
At line 82 added 32 lines
Numerous tests are made within the assembler for user errors:
# All parameters used in CODE definitions must be removed.
# Conditionals must be properly nested and paired.
# Address modes and operands must be allowable for the op-codes
These tests are accomplished by checking the stack position (in CSP) at the creation of the definition name and comparing it with the position at END-CODE. Legality of address modes and operands is insured by means of a bit mask associated with each operand.
Remember that if an error occurs during assembly, END-CODE never executes. The result is that the "smudged" condition of the definition name remains in the "smudged" condition and will not be found during dictionary searches.
The user should be aware that one error not trapped is referencing a definition in the wrong vocabulary:
i.e., {{{0=}}} of ASSEMRLER when you want {{{0=}}} of FORTH
(Editor's note: the listing assumes that the figFORTH error messages are already available in the system, as follows:
?CSP issues the error message "DEFINITION NOT FINISHED" if the stack position differs from the value saved in the user variable CSP, which is set at the creation of the definition name.
?PAIRS issues the error message "CONDITIONALS NOT IMPAIRED" if its two arguments do not match.
3 ERROR prints the error message "HAS INCORRECT ADDRESS MODE".)
!!SUMMARY
The object code of our example is:
{{{
3059 83 4D 4F CE CODE MON
305D 4D 30 link field
305F 61 30 code field
3061 00 BRK
3062 4C 42 02 JMP NEXT
}}}
This page (revision-7) was last changed on 03-Feb-2023 15:21 by Carsten Strotmann  Top