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 Post subject: NMI on a Vic-20
PostPosted: Sun Aug 25, 2019 9:17 pm 
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Joined: Wed Mar 02, 2016 12:00 pm
Posts: 343
Hi

The NMI on the Vic-20 is connected directly to the "RESTORE" key on the keyboard. Not much happens if you press it since the routine just returns to the interrupt break point with a RTI. That is unless the "RUN STOP" key is pressed. In that case, it does not return to the interrupt breaking point. It seems like the Commodore Basic at this point clears the stack by writing $FA into the stack pointer.

During Basic cold start, it clears the stack by writing $FB into the stack pointer.

During reset, it clears the stack by writing $FF into the stack pointer (which kind of makes more sense).

The question is why the "Kernal" writes $FB or $FA into the stack pointer during different cold starts. It retains something there, but I can't understand what.

I don't know how much of this code is Commodore specific since its basically MS Basic v2.0.


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 Post subject: Re: NMI on a Vic-20
PostPosted: Mon Aug 26, 2019 12:32 am 
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Joined: Sun Jun 30, 2013 10:26 pm
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Location: Sacramento, CA, USA
Although I am not by any means an 8-bit Commodore expert, I can readily postulate that the behavior you mention is completely Commodore-specific, since there is not a single "RTI" in nearly 7000 lines of the original 6502 source, implying to me that MSBASIC has no stake in any interrupt handling. Why the "Kernal" initializes the stack pointer differently during different cold starts is a question whose answer is beyond my pay grade. ;-)

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 Post subject: Re: NMI on a Vic-20
PostPosted: Mon Aug 26, 2019 1:33 am 
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IIRC, the Vicmon cartridge used the NMI to perform single step execution, and it might have been used for the RS-232 implementation using the 6522 VIA. Fortunately, a disassembly of the ROMs are available which you will likely find useful and enlightening.

http://www.zimmers.net/anonftp/pub/cbm/ ... index.html

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 Post subject: Re: NMI on a Vic-20
PostPosted: Mon Aug 26, 2019 5:50 am 
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Posts: 21
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kakemoms wrote:
During Basic cold start, it clears the stack by writing $FB into the stack pointer.

In the C64, Commodore calls this a warm start. It restarts BASIC and the shell, but not the kernel (or kernal in Commodore parlance).
The top of the stack me already be in use by the kernel, so it should not be overwritten.

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 Post subject: Re: NMI on a Vic-20
PostPosted: Mon Aug 26, 2019 6:21 am 
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floobydust wrote:
... a disassembly of the ROMs are available which you will likely find useful and enlightening.

http://www.zimmers.net/anonftp/pub/cbm/ ... index.html


Thanks! It looks like the top of page 1 is used for storage, which might help explain why the stack is (sometimes) initialised to a different value:

Code:
LAB_01FC   = $01FC      ; chain link pointer high byte
LAB_01FD   = $01FD      ; chain link pointer low byte

LAB_01FE   = $01FE      ; line number low byte before crunched line
LAB_01FF   = $01FF      ; line number high byte before crunched line


Here are the three initialisations:

Code:
; flush BASIC stack and clear the continue pointer

LAB_C67A
   LDX   #LAB_19      ; get descriptor stack start
   STX   LAB_16      ; set descriptor stack pointer
   PLA            ; pull return address low byte
   TAY            ; copy it
   PLA            ; pull return address high byte
   LDX   #$FA         ; set cleared stack pointer
   TXS            ; set stack
   PHA            ; push return address high byte
   TYA            ; restore return address low byte
   PHA            ; push return address low byte
   LDA   #$00         ; clear A
   STA   LAB_3E      ; clear continue pointer high byte
   STA   LAB_10      ; clear subscript/FNX flag
LAB_C68D
   RTS


Code:
; BASIC cold start entry point

LAB_E378
   JSR   LAB_E45B      ; initialise BASIC vector table
   JSR   LAB_E3A4      ; initialise BASIC RAM locations
   JSR   LAB_E404      ; print start up message and initialise memory pointers
   LDX   #$FB         ; value for start stack
   TXS            ; set stack pointer
   JMP   LAB_C474      ; do "READY." warm start


Code:
; RESET, hardware reset starts here

LAB_FD22
   LDX   #$FF         ; set X for stack
   SEI            ; disable interrupts
   TXS            ; clear stack
   CLD            ; clear decimal mode
   JSR   LAB_FD3F      ; scan for autostart ROM at $A000
   BNE   LAB_FD2F      ; if not there continue Vic startup

   JMP   (LAB_A000)      ; call ROM start code

LAB_FD2F
   JSR   LAB_FD8D      ; initialise and test RAM
   JSR   LAB_FD52      ; restore default I/O vectors
   JSR   LAB_FDF9      ; initialize I/O registers
   JSR   LAB_E518      ; initialise hardware
   CLI            ; enable interrupts
   JMP   (LAB_C000)      ; execute BASIC


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