SAGE: fix thirteen 68000 bugs that kept the UCSD p-System from booting - #579
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SAGE: fix thirteen 68000 bugs that kept the UCSD p-System from booting#579ajfa wants to merge 1 commit into
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The Sage II simulator has only ever been exercised by CP/M-68K, and the readme lists the UCSD p-System as not working yet. Trying to boot it turns up thirteen defects in the 68000 core. With these fixes the four Sage p-System IV.13 diskettes and the four Stride IV.21 diskettes reach their command prompts, the Filer lists both drives, and CP/M-68K 1.2 still boots exactly as before. What was wrong: * MOVE.B/.W/.L did not clear V and C. * CMP, CMPA and CMPI wrote the X flag, which they must leave untouched. * CMPA.W compared a truncated address register against an unextended source. * CMPM.B/.W/.L were not implemented. * SUBX had its operands the wrong way round in all six forms. * MOVE to SR, the immediate forms of ANDI/ORI/EORI to SR, and STOP did not swap A7 with USP when the S bit changed, so a program that dropped to user mode carried on pushing onto the supervisor stack. * The 8 bit displacement of the (d8,An,Xn) modes was sign extended as if it were 16 bits, in all eleven places that build such an address. Every negative displacement therefore reached the wrong memory. * OR.L <ea>,Dn computed an AND. * OR.W <ea>,Dn stored its result as a byte. * DIVS.W never fetched its source operand and used whatever the previous instruction had left in the temporary. * DIVS.W divided by the raw unsigned divisor instead of the sign extended one. * The bit mask table had four wrong entries, so BTST, BSET, BCLR and BCHG worked on the wrong bit whenever the bit number was 15, 19, 23 or 27. * ROXR.L and ROXL.L loaded a signed int32 into their 64 bit temporary, which carried the sign into the rotated result. How they were found: a differential tester runs one random instruction at a time on this core and on Musashi from the same starting state, then compares the sixteen registers, the PC, the condition codes and a 64 byte window of memory. Out of 425 comparable cases, 98 disagreed before these fixes and 8 after. Five of those eight are cases where the two models take a different view of addresses outside the simulated 512 KB. The remaining three are the carry flag of ROR.B and ROR.L and the flags of CHK.W, which are still open. Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
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Similar comment about identifying who you are mentioned in simh/simh#1256. Otherwise, since you're making a good jump at fixing things here in a simulator which, seems to have had a very quiet life in the 12 years it has been in the codebase, why stop before addressing the remaining 8 cases you found to be unexpected? |
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The Sage II simulator only ever gets exercised by CP/M-68K, and
readme-sage.txtstill lists UCSD Pascal as "does not run yet". It turns out the p-System is blocked by thirteen separate bugs in the 68000 core rather than by anything specific to the Sage hardware.With this patch:
Command: E(dit, R(un, F(ile, ...and the Filer lists both drivesThe commit message lists all thirteen. The three with the widest reach are probably these:
MOVEnever cleared V and C, which accounts for most of the difference on its own(d8,An,Xn)modes sign extended the 8 bit displacement as if it were 16 bits, in all eleven places that build such an address, so every negative displacement reached the wrong memoryBTST,BSET,BCLRandBCHGworked on the wrong bit whenever the bit number was 15, 19, 23 or 27How this was tested
A differential tester runs one random instruction at a time on this core and on Musashi from the same starting state, then compares the sixteen registers, the PC, the condition codes and a 64 byte window of memory. The SIMH side is driven through the simulator's own command interface, with
deposit,step 1andexamine, so what gets tested is the simulator as built rather than a relinked copy of the core.Out of 425 comparable cases in a run of 800 random instructions, 98 disagreed before these fixes and 8 after. Five of those eight are cases where the two models take a different view of addresses outside the simulated 512 KB. The other three are real and still open: the carry flag of
ROR.BandROR.L, and the flags ofCHK.W.The p-System images used for the boot tests are David Erhart's ImageDisk dumps of original Sage and Stride diskettes. They contain compressed sectors, so they need uncompressing before the simulator will mount them read/write.
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