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Old 13 October 2020, 23:05   #1
Bruce Abbott
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What instruction is this? $F041 $0001

Devpac 3 and Barfly debuggers both refuse to disassemble the instruction shown in hex below (partial disassembly of ProAsm V1.92). Does anyone know what it is?

Code runs in supervisor mode and appears to be trying to determine the processor type or features.

Code:
 cmp.w   #$0004,D2
 beq.s   w001570
 .
 .
w001570:
 dc.w    $f041      <--- unknown instruction
 dc.w    $0001
 nop
 nop
 or.l    D0,D2
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Old 13 October 2020, 23:22   #2
DanScott
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I don't think it's a vanilla 68000 instruction ?
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Old 13 October 2020, 23:35   #3
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It's a 68851 mmu instruction (psbc d1).
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Old 13 October 2020, 23:36   #4
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WinUAE disassembles it as Illegal Instruction.

Action Replay disassembles it as PSBC D1

Very odd - not seen that before.
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Old 13 October 2020, 23:50   #5
ross
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Confirmed in qemu source:
{"psbc", 4, two(0xf040, 0x0001), two(0xffc0, 0xffff), "$s", m68851 },
0xf040->d0, 0xf041->d1
and MC68851 manual (page 265)
cc second instruction word
BC B clear 000001 word


Yes, PSBC D1 instruction.
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Old 14 October 2020, 22:47   #6
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Damn this is a rare chip I had never heard of before. Were there Amigas with 68020 and MC 68851 as MMU? I doubt WinUAE supports this either.
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Old 14 October 2020, 23:25   #7
phx
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Quote:
Originally Posted by jotd View Post
Were there Amigas with 68020 and MC 68851 as MMU?
Sure. I have an A2500 which came with 020/851/881. It's the ultimate testing target!
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Old 14 October 2020, 23:32   #8
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The A2620 could use the 68851, but I'm not sure if it was equipped as standard or optional.
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Old 14 October 2020, 23:54   #9
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I'm sure Toni will be delighted to support that chip in WinUAE, and Wepl will be thrilled to support that chip in Whdload
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Old 15 October 2020, 17:29   #10
Toni Wilen
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I don't think there are any Amiga programs that require 68851 or use 68851 features. First requirement for emulation some hardware: it needs software!
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Old 15 October 2020, 18:20   #11
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Quote:
Originally Posted by Toni Wilen View Post
I don't think there are any Amiga programs that require 68851 or use 68851 features. First requirement for emulation some hardware: it needs software!
The original Guardian Angel by Valentin Papaela(sp?).
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Old 15 October 2020, 18:22   #12
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Quote:
Originally Posted by Toni Wilen View Post
I don't think there are any Amiga programs that require 68851 or use 68851 features. First requirement for emulation some hardware: it needs software!
Enforcer?
http://www.sinz.org/Michael.Sinz/Enforcer/index.html

OpenBSD?

https://www.openbsd.org/amiga.html
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Old 15 October 2020, 18:33   #13
Toni Wilen
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Do they actually use 68851-only features?

If program also supports 68030, modifying it to work with 68020+68851 does not require use of any 68851-only features. (68030 MMU is more or less integrated 68851 with lots of weird features dropped)
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Old 15 October 2020, 18:49   #14
Don_Adan
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Quote:
Originally Posted by Toni Wilen View Post
Do they actually use 68851-only features?

If program also supports 68030, modifying it to work with 68020+68851 does not require use of any 68851-only features. (68030 MMU is more or less integrated 68851 with lots of weird features dropped)
Enforcer seems not:

* 68020 NOTES
* The 68020 does not have a built-in MMU but has a co-processor
* feature that lets an external MMU be connected. Enforcer MMU code
* is designed for use with 68851 MMU. This is the some-what 68030
* compatible MMU by Motorola. Enforcer uses the same code for both
* the 68030 and the 68020/68851. For this reason, 68020/68851 users
* should see the 68030 NOTES section.
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Old 15 October 2020, 21:01   #15
DrBong
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Quote:
Originally Posted by Toni Wilen View Post
I don't think there are any Amiga programs that require 68851 or use 68851 features. First requirement for emulation some hardware: it needs software!
Quote:
Originally Posted by Toni Wilen View Post
Do they actually use 68851-only features?

If program also supports 68030, modifying it to work with 68020+68851 does not require use of any 68851-only features. (68030 MMU is more or less integrated 68851 with lots of weird features dropped)
Virtual memory utils were perhaps the most well-known mainstream use of the MMU on the Amiga (e.g. VMM, Gigamem, HDMem). Other software like Shapeshifter/Fusion, AMIX/NetBSD, and PhxAss are able to make significant use of the MMU, so likely support the 68851 too. Whether they used any features unique to the 68851 is hard to know without perhaps looking at the code closely because, as you say, the 68030 MMU supports many features of the 68851. It wouldn't make great sense for the coders of these utils to support something unique to the 68851 to the exclusion of the MMUs built-in to higher CPUs, but you never know.

Last edited by DrBong; 15 October 2020 at 21:51. Reason: Added more links!
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Old 16 October 2020, 16:05   #16
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AMIX (Amiga UNIX) uses 68020+68851 or 68030 with MMU. Also WHDLoad can use MMU partially. (You know that there was 68451 MMU for 68000/010, but it was rarely used.)

There are 2 cpScc (cp = coprocessor) instructions: PScc (68851) and FScc (FPU; native on 68881, 68882 and 68040; software emulated on 68060 - in 68060.library on Amiga).
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Old 16 October 2020, 19:58   #17
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As NetBSD & AMIX are here I might we can add Linux/68k which Geert Uytterhoeven still seems to be supporting to this day.
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Old 16 October 2020, 20:00   #18
Toni Wilen
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Repeat reply: I don't think they use any 68851-only features.
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Old 16 October 2020, 20:13   #19
phx
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Toni is probably right. The 851-specific features are rarely used. And I can confirm for NetBSD that 68851 and 68030 systems are treated the same.

Quote:
Originally Posted by DrBong View Post
and PhxAss are able to make significant use of the MMU
Wow! I didn't know that.
(It can assemble 68851 mnemonics as many other assemblers do.)
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Old 16 October 2020, 23:18   #20
DrBong
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Quote:
Originally Posted by phx View Post
Toni is probably right. The 851-specific features are rarely used. And I can confirm for NetBSD that 68851 and 68030 systems are treated the same.
Even if he wasn't, what could possibly warrant the time & energy spent on supporting obscure 68851-specific features under emulation?

Quote:
Wow! I didn't know that.
(It can assemble 68851 mnemonics as many other assemblers do.)
Yeh, but only when you use assemblers on a military-spec Amiga 2000! Anyway, it was interesting to learn that even the 68010 had the possibility of adding an external MMU (68841)....but no home computer used it AFAIK.

Last edited by DrBong; 16 October 2020 at 23:24.
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