History

Bobbi Webber-Manners fb2d438958 Update README.md		2021-06-05 14:47:45 -04:00
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README.md	Update README.md	2021-06-05 14:47:45 -04:00
build_nsc	Improved build script. Changed file naming.	2021-05-29 21:12:05 -04:00
nsc.s	Modified NSC driver for use with Ultrawarp	2021-05-29 20:54:07 -04:00
nsc_ultrawarp.po	Improved build script. Changed file naming.	2021-05-29 21:12:05 -04:00
nsc_ultrawarp.s	Modified NSC driver for use with Ultrawarp	2021-05-29 20:54:07 -04:00

README.md

`NS.CLKUW.SYSTEM`

No Slot Clock Driver for ProDOS

NS.CLKUW.SYSTEM is a version of the original No Slot Clock Driver NS.CLOCK.SYSTEM, modified to work with the RM Ultrawarp accelerator.

nsc.s is the original No Slot Clock driver written by SMT, which has been disassembled and commented by Github user mgcaret. I took it from here.

The RM Ultrawarp Apple II accelerator card has some sort of bug in the banking of ROMs. As a result of this problem, the original NS.CLOCK.SYSTEM does not work on a system with Ultrawarp, unless the accelerator card is completely disabled. The symptom is that the NSC is detected and the ProDOS driver is installed, but the time returned is all zeroes. It does not appear to be a problem related to the 13MHz clock speed of the Ultrawarp accelerator because it also manifests if the Ultrawarp is active but with 1MHz clock.

nsc_ultrawarp.s is a modified version of nsc.s which includes a workaround for this problem. It has the following differences compared to nsc.s:

The original code searches for an NSC installed under peripheral card ROMs in each of the Apple II slots, and only then searches for a NSC installed in the normal place, which is underneath the CE or CX ROM on an Apple //e, or the Monitor ROM on an original Apple II or plus. I removed the search through the slots, and now only look for an NSC installed under the CE/CX ROM or Monitor ROM.
In the original code, the driver code itself is modified in place to work with the NSC in the location where it was detected. This is no longer required, so I removed the self-modifying code. This makes the code easier to read, and also makes the clock driver a few bytes shorter, which is critical, as there was no space to add any additional code.
Added an additional instruction LDA $C00B at the beginning of the clock driver. This causes the driver to work properly with the Ultrawarp active, either at full speed (13MHz) or 1MHz. The reason adding this one read fixes the erroneous behaviour is not well understood. It seems that the additional instruction causes the CE/CX ROM to be banked in. Without this extra instruction, it seems the ROM was not banked in when running with the Ultrawarp. This appears to be a hardware bug in the Ultrawarp.

Bobbi May 29, 2021