Re: [fuse-emulator-devel] Disciple Support on 128K Spectrum

[Szász G. <sz...@hu...>]

Hi Alan,

Could you publish the PAL equivalent logic in https://sinclair.wiki.zxnet.co.uk

e.g. on the https://sinclair.wiki.zxnet.co.uk/wiki/DISCiPLE page or as an "Integrated Circuit" https://sinclair.wiki.zxnet.co.uk/wiki/Category:Integrated_Circuit

best regards,
gergely

:)

On Tue, Mar 05, 2024 at 11:01:48PM +0000, Alan Pearson wrote:
> Hi All,
> 
> 
> I'm the guy wrote 'The Disciple PALs finally unveiled' and I found this thread discussing the issue on WoS by accident while trying to figure out why the disciple isn't working on FUSE in 128K mode.
> https://worldofspectrum.org/forums/discussion/42567/the-disciple-clone-doable/p1???
> What an interesting thread , and i was shocked to see my doc referenced.
> I haven???t been able to create an account on WoS to reply there, it seems the admin may have gone on holiday or something???. so I came here.
> 
> First, the document:
> 1) This was a direct reverse engineer of working PAL chips, put on a logic analyser by Rudy Biesma, then turned into the equations manually by myself then jedec files created.
> 2) All the findings & equations seem to correspond to known info about the disciple (manual, Ramsoft guide and Rudy's disassembly)
> 3) The proof however was in real world testing - I repaired a broken disciple by programming the PAL chips with the jedec files created and it works perfectly on a 128k +2 (grey).
> 4) There is a typo... N1 should be M1 throughout the doc, but I think you guys worked that out already
> 
> Now, from my reading of the WoS thread and the Rudy disassembly, it seems there is no way the disciple should work on a 128K (at least no way it should initialise)... but it does.
> The code path just doesn't make sense on a 128K machine at reset time.
> 
> Let's change tact - the PlusD uses EXACTLY the same initialisation code, lands in the same place in the ROM when it unpages itself at reset (location #52) and makes no attempt (on reset at least) to figure out if it's a 1128K or 48K or 128k spectrum or which spectrum rom is in.
> How does the plusD then work under FUSE ? It's very weird.
> It's worth saying that the PlusD does have routines specific to 128K paging/unpaging but they are only used when the COPY command finishes and the machine resets after completion.
> The PlusD TO_NEW routine essentially jumps to location #C7 on a 128k (reset routine in the editor ROM) or #11B7 in 48K. 
> Relevant routines below, but as I said they are NOT called when the system resets.
> 
> From Rudy's +D disassembly :
> 
> THE 'UNPAGE 128 ROM' SUBROUTINE
> This routine pages out the +D system and returns to the 128K ROM at address #0049. That
> address contains a RET instruction so the effect is a jump to BC with the 128K ROM paged
> in. 
> 
> 0046 UNPAGE_BC  PUSH BC                  Stack return address.
> 0047 UNPAGE_0   OUT  (231),A             Page out +D system.
> 0049            RET                      ? This statement is not reached from
>                                          above.
> 004A            DEFB #00,#00,#00,#00,#00 Unused locations.
> 
> THE 'NEW' PATCH
> When the file copy command is finished the +D jumps to the 'NEW' routine. With System 2a
> a selection is made for 48K or 128K 'NEW'
> 
> 3137 TO_NEW     BIT  4,(IY+1)            Jump if not in 128K mode HL contains
> 313B            JP   Z,#004F,UNPAGE_HL   #11B7, the address of 48K 'NEW'.
> 313E            CALL #5B00,SWAP          Call the paging subroutine of the 128.
> 3141            DI
> 3142            LD   BC,#00C7            Address of 128 'NEW' routine.
> 3145            JP   #0046,UNPAGE_BC
> 
> 
> 
> Regarding the PALs, I suspect the equations are right, and the fact the code exists in both the +d and Disciple to initialise them suggests that it should be used, so I don't think this is a problem of paging in the interfaces at the wrong time. That said, I can't see how it all should work on a 128, but it does. So this can't be a disciple specific problem, can we trace the +D codepath in FUSE in  128K mode?? I'm not sure how to do this myself.
> 
> 
> It would be great to see Disciple working in 128K models... there are difference between the interfaces that I've run into in converting a program to load from disc (specifically the interface 1 hook code #32, which does a RET on the disciple, but on the PlusD allows any ROM addresss to be called via DE register)
> 
> I know this is a horrible kludge, but can we not monitor location #52 on both inrwefaces and if we have a 128K ROM paged in, simply jump to #00C7 after the OUT instruction ?
> 
> 
> 
> Any thoughts ?
> 
> Alan


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