Anachronaut
87d819847e
SWI osExit takes a status in A, and the shell keeps it. Fifty eight exits across twenty three programs now say deliberately whether they worked: 25 did what they were asked, 24 did not, 9 were asked wrongly. Compare is the exception and says so - one there means the files differ, which is a result rather than a failure, the way diff has always had it. IN A RATHER THAN Q, which is not a departure from the rule that a service answers in Q. This one takes an ARGUMENT, the way osPrintNumber takes A and B, and it never returns to answer anything. A is free precisely because a return would have put it back - and Q is the ALU's output, so a small number costs four instructions there against one in A. The shell does not print it. A program that failed has already said so in words and a number beside that is noise, so osLastStatus hands it back and Status is the program that shows it. That indirection is the point: the number exists for the thing that cannot read words. MARKING THE EXITS FOUND A DEFECT ON THE FIRST RUN. Type and More printed why they had failed and then fell through into the success exit, reporting that all was well. Nobody had noticed, because while the only reader was a person, the person could see both the complaint and the claim. Two smaller things. Snake sets the console to line mode and then exits with zero, and the linter flagged the second RSTA as redundant - an exit status and a console mode, equal by accident, which is the class that must never be collapsed. And the README still taught answering by writing into the frame, three months of habit that SRET replaced yesterday; that section is gone and the one describing SRET stands in its place.
85 lines
2.7 KiB
NASM
85 lines
2.7 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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;
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; EVERY POINTER IS SET BEFORE EACH STOP, and the three this program owns are rotated
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; between the two so that all of them visibly change.
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;
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; This program used to leave DP1 and DP2 alone, and what a stop then showed for them was
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; whatever the shell happened to have left there. That is a real thing about the machine -
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; a program is handed the pointers as it finds them - but it is not this program's to
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; demonstrate, and it made what this program prints depend on where CosmOS's code happens
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; to sit. The recorded output had to be taken again four times in one day's work, every
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; time for a value nothing should ever depend on.
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;
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; A demonstration of what the registers were should show registers somebody chose. Then
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; every line of it is being asserted rather than merely observed.
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;
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; DP3 is deliberately still the system's. It is where the program was entered, which is the
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; one thing here worth seeing that this program did not choose, and it is steady because it
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; is this program's own base.
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#Include services.asm
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#Program
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#Base 0x4000
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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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SETD.1 Banner
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SETD.2 DoneText
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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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RSTA
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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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SETD.1 DoneText
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SETD.2 Marker
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SWI osBreak
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RET
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#Data
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#Base 0x2000
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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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