[Flashforth-devel] PIC18F - Problems with code above $8000

[Pablo M. <ea...@ho...>]

Hello Mikael,


I am writing a relatively big (o lousy coded) program, and have found that words created above $8000 sometimes result in a processor reset when compiled into new words.


Here is a simple example:

hex flash here u. 895a  ok<$,flash>
ram   ok<$,ram>
: test1 1 1 + ;    ok<$,ram>
test1  ok<$,ram>2
: test2 test1 ;  ok<$,ram>2
test2
 FlashForth 5 PIC18F26K20 07.10.2017     -->   [test2 has produced a reset]

After a lot of head scratching, I have found that CALL and GOTO instructions have an incorrect destination address when the target is above $8000:
' test1  ok<$,ram>8962

hex see test2
897a efb1 f0c4 goto  test1

The destination addresss should be 44b1 (= 8962 / 2), but the compiled address is c4b1. This results in a jump outside the program.

The problem is that the destination address is calculated with the 2/ code (TWOSLASH routine). But 2/ is designed to work with signed numbers. It does so by shifting 1 bit right but maintaining the sign bit.

8962 2/  ok<$,ram>c4b1                  <--- This is the used (incorrect) address
8962 1 rshift  ok<$,ram>c4b1 44b1       <--- This is the correct address

The simplest solution would be to change 2/ to work with unsigned numbers. But I see that the current implementation of 2/ is correct according to the ANS94 standard, so it doesn't seem right to change it.

Perhaps the best solution is to create a new subrutine "u2/"
UTWOSLASH:
        bcf     STATUS, C, A
        rrcf    Sminus, F, A
        rrcf    Splus, F, A
        return
and use it instead of TWOSLASH in "br?" and "br3".



   73, Pablo - EA4FUK