AnachronautandClaude Opus 5
c23adb2836
The dump labelled the status register "S", which reads as Stack to anybody sensible - and the Stack Pointer was the one register it did not show, so there was nothing to contradict the guess. It is written "status" now, and followed by the bits that are up, because a dump that makes you look the number up is only half a dump. The Stack Pointer is not in the frame, since the frame is where the Stack Pointer is. What the program had is fourteen bytes above it, that being what entering an interrupt puts down, so it is worked out and shown. Apps/Break.asm takes its second stop inside a subroutine, so the recorded output shows the Stack Pointer at FFFF and then at FFF5: a difference of ten, which is the size of a CALL frame. That checks the value is derived rather than constant, which the previous version could not have told you. Reported by Anachronaut, who read the output and asked why a pointer was two digits long. Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
63 lines
1.7 KiB
NASM
63 lines
1.7 KiB
NASM
; Stopping a program to look at it.
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;
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; SWI osBreak shows every register as this program had them, waits for a key, and carries
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; on. It is two bytes and it fires every time it is reached.
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;
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; WHY IT IS AN INSTRUCTION RATHER THAN SOMETHING SET FROM OUTSIDE. A breakpoint that was
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; poked into a running program would have to overwrite an instruction, and then putting that
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; instruction back in order to continue is the same act as disarming the breakpoint. Firing
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; a second time would mean stepping over the restored instruction and putting the breakpoint
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; back behind it, and this machine cannot step one instruction. Nothing is overwritten here,
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; so there is nothing to restore and nothing to re-arm.
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;
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; The price is that it is part of the program. A build with breakpoints in it has different
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; addresses from a build without, which is the same bargain every machine makes that has a
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; break instruction.
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;
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; Correct output is two stops. A and B differ between them, the addresses differ, and the
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; Stack Pointer differs too, because the second one is inside a subroutine and a call has
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; put ten bytes down by then.
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#Include services.asm
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#Program
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#Base 0x2000
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start:
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SETD.0 Banner
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SWI osPrintString
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INIA 0d17
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INIB 0d34
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SETD.0 Marker
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SWI osBreak
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; The second stop is inside a subroutine, so that the Stack Pointer is visibly not where
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; it was: a call puts ten bytes down before this one gets there.
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CALL deeper
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SETD.0 DoneText
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SWI osPrintString
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SWI osExit
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deeper:
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INIA 0d68
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INIB 0d85
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SETD.0 Banner
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SWI osBreak
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RET
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#Data
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#Base 0x1000
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Banner:
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"two stops, and what the registers were at each
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"
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Marker:
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"marker"
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DoneText:
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"carried on to the end
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"
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