PET startup sequence?? (was LF: Commodore PET schematics, troubleshooting info)

[Dave Dunfield]

A little more progress...

Here is the BASIC startup code, which is jumped to by the kernel (in F000 ROM) if DIAG is
not pulled low (Basically, it goes either here or the monitor):

; initcz        Initialize BASIC RAM
 D3B6   iD3B6   LDX #$FB
 D3B8           TXS
 D3B9           LDA #$4C
 D3BB           STA $51
 D3BD           STA $00         ; USR Function Jump Instr (4C)
 D3BF           LDA #$73
 D3C1           LDY #$C3
 D3C3           STA $01         ; USR Address [4: C373]
 D3C5           STY $02
 D3C7           LDX #$1C
 D3C9   iD3C9   LDA $D398,X
 D3CC           STA $6F,X
 D3CE           DEX
 D3CF           BNE $D3C9
 D3D1           LDA #$03
 D3D3           STA $50
 D3D5           TXA
 D3D6           STA $65         ; Floating -accum. #1: Overflow Digit
 D3D8           STA $10         ; 3: width of source (unused - from TTY)
 D3DA           STA $15
 D3DC           STA $0D         ; 3: Flag to suppress PRINT or PRINT#
 D3DE           PHA
 D3DF           INX
 D3E0           STX $01FD
 D3E3           STX $01FC
 D3E6           LDX #$16
 D3E8           STX $13         ; Pointer Temporary String
 D3EA           LDY #$04
 D3EC           STA $28         ; Pointer: Start of BASIC Text [0401]
 D3EE           STY $29
 D3F0           STA $11         ; Temp: Integer Value
 D3F2           STY $12
 D3F4           TAY
 D3F5           LDA #$80
 D3F7           BNE $D400
 D3F9           LDA #$00
 D3FB           LDY #$B0
 D3FD           JMP $D41B
 D400   iD400   INC $11         ; Temp: Integer Value
 D402           BNE $D408
 D404           INC $12
 D406           BMI $D417       ; initms        Output Power-Up Message
 D408   iD408   LDA #$55
 D40A           STA ($11),Y     ; Temp: Integer Value
 D40C           CMP ($11),Y     ; Temp: Integer Value
 D40E           BNE $D417       ; initms        Output Power-Up Message
 D410           ASL
 D411           STA ($11),Y     ; Temp: Integer Value
 D413           CMP ($11),Y     ; Temp: Integer Value
 D415           BEQ $D400

; initms        Output Power-Up Message
 D417   iD417   LDA $11         ; Temp: Integer Value


On the working PET, I can use 'G D3B6' and it starts BASIC, just as if it
were running normally. On the bad PET, it hangs, and never prints the message
which is output by the very next block (D417).

[Interesting side note - this NEVER produces the startup "noise", so it is
clearly not produced by BASIC either ... a mystery]

Now, on the bad pet, if I do 'G D417', which bypasses this block of code,
it issues the startup message and comes up in BASIC (somewhat weird because
various things were not setup, but it's there).

So - thats pretty conclusive proof that this is the block where the problem
is manifesting itself.

Looking at the block of code, it "shouldn't" hang ... There are only two
loops, the first is counted by a register (as long as the CPU is good this
should work, and I have swapped the 6502 with the other machine) ... the
second goes until RAM fails to verify (this will definately happen by the
time it reaches the ROM), or the $11,$12 location wraps to a negative
value (32k max RAM). There are no subroutine calls in this block, so it
should not depend on a valid stack.

Using my "try BASIC then reset to monitor without interrupting power"
technique, I see that $11,$12 almost always contains $01,$40, which means
that it reached the INC $11 once (first time through).

The only way I can see this loop crashing is if the $11 (or $12) location
does no increment correctly (RAM fauly) - I've manually tested all kinds of
values in them, or if the ROM "goes bad" during code execution.

[If the $12 location was faulty, I should see "random" values in $11 depending
on the timing of when I hit reset]

I'm thinking ROM again (this was my "gut feel" when I started because I have
seen so many PET ROM's go bad) - although it reads OK in my EPROM programmer,
and also this block dumps correctly from the monitor, both of these accesses
are fairly laid back - hitting the code is the first time we execute code from
this device... Timing on execute could be tighter (I haven't checked the
databook yet), also the frequency of accesses will be much higher ... Perhaps
if the ROM is marginal, one of these factors is enough to cause a failure.

I think a bad ROM is more likely than a single location in the RAM appearing
to work for manual tests, but failing during this particular block of code,
especially considering that the rest of RAM is good enough for lots of other
stuff to run ...

What do you guys think?  Anyone have experiences with marginal PET roms?

Regards,
Dave

-- 
dave04a (at)    Dave Dunfield
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com             Vintage computing equipment collector.
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