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Really super weird things about the Lisa
sigma7:
Another weird thing is that the Lisa design does not support the 68K's read-modify-write cycle.
The read-modify-write cycle is atomic so it can be used to lock a resource at the same time as checking it is free without fear of a perfectly timed interrupt creating a contention problem between processes. The TAS "Test And Set" instruction is the only 68000 instruction that uses the RMW cycle.
The discovery of this limitation was triggered by a SCSI connected Floppy drive by PLI; their "Turbo Floppy" aka "Super Floppy". This drive was designed to bring compatibility with Apple's 1.4MB SuperDrive to the Mac Plus. It doesn't work on a stock Lisa, but does work when an XLerator is used.
The PLI drive enclosure contains a ROM that provides the appropriate SCSI driver to the computer. However, the driver uses the TAS "Test And Set" instruction. The driver doesn't use this instruction for its special RMW feature, but it does save a few bytes.
It was a surprise to discover that the Lisa didn't support the TAS instruction, as one would expect other software would have used this instruction before, revealing this issue much sooner in the MacWorks life cycle.
The XLerator does support the RMW cycle, so when it is in use, the SCSI floppy works.
patrick:
--- Quote from: patrick on July 08, 2019, 04:54:45 pm ---
--- Quote from: rayarachelian on July 05, 2019, 10:53:07 am ---Most likely nothing will happen. But it would be an interesting experiment to see what codes the COP421 produces when these extra two mouse buttons are triggered, if any. (It may well be that while the hardware is still there that Apple removed the code from the COP421, in which case, you could never take advantage of the extra buttons.)
--- End quote ---
If someone wants to investigate this in detail: here is the COP421-HZT ROM code: http://bitsavers.org/bits/Apple/Lisa/firmware/COP421-HZT_LisaIO.zip
To test modified COP firmware in your Lisa, you will have to obtain a ROMless COP402 and wire a circuit like this: http://john.ccac.rwth-aachen.de:8000/patrick/COPSreader.htm#emulator
--- End quote ---
Another note on COPS replacements: The National COP402, which is difficult to obtain today, can be substituted by the Russian KR1820VE1A (in cyrillic letters "KP1820BE1A"). These are sold quite often on ebay. However, this part has a metric 2.50 mm pitch instead of the common 2.54 mm (0.1"). Therefore it does not fit into "precision" sockets with turned contacts. "Cheap" sockets with straight spring contacts forgive the missing 0.76 mm, and direct soldering into the PCB will also work.
A National COP402LSN-5 will not work with the Lisa. This is a low-power version with an internal clock divider of /32 instead of /16.
Patrick
Lisa2:
--- Quote from: patrick on September 28, 2019, 11:03:00 am ---The National COP402, which is difficult to obtain today, can be substituted by the Russian KR1820VE1A (in cyrillic letters "KP1820BE1A").
--- End quote ---
Thanks for the information Patrick! For some reason I am fascinated by Russian Clone chips..
Rick
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