wbell@utgpu.UUCP (03/16/87)
Has anyone experienced any compatibility problems with the SE?
Do any programs which work on the Plus fail in strange ways?
(I heard something about some DA's not working properly due to its new ROMs)
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University of Toronto Computing Services Warren Bell
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Disclaimer: the views expressed above are mine alone, and do not
in any way reflect the views of the University of Toronto, nor
the views of University of Toronto Computing Services.eacj@batcomputer.tn.cornell.edu (Julian Vrieslander) (03/20/87)
I have encountered a pretty arcane incompatibility between the Plus and the SE.
On the Plus, you can read bit 6 in VIA register B to get at the horizontal synch
signal, which is also the clock for the sound generator DAC. I have been using
this bit as a time base for a very tricky custom sound driver.
In the SE, that bit no longer follows the horizontal retrace (I think it is used
for reporting disk drive interrupts now). In fact, I remember seeing somewhere
that the VIA chip used in the SE is a special version of the SY6522, modified
for Apple.
I was expecting to write a new driver for the Mac II, but I was surprised to
find that my program died on an SE. Is there any other place in the SE that
software can read the status of the horizontal retrace? Or should I warm up my
soldering iron?
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Julian Vrieslander, Neurobiology & Behavior, Cornell Univ., Ithaca, NY 14853
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ARPA: eacj@tcgould.tn.cornell.edu MA BELL: (607) 255-3594jww@sdcsvax.UUCP (03/21/87)
In article <458@batcomputer.tn.cornell.edu>, eacj@batcomputer.tn.cornell.edu (Julian Vrieslander) writes: > On the Plus, you can read bit 6 in VIA register B to get at the horizontal synch > signal, which is also the clock for the sound generator DAC. The Mac II has a vertical retrace every 1/67th of a second. Quoting from IM Draft Volume V, ch 29: 'With the advent of slots, a variety of screens are available, each with different vertical retrace periods. ... For compatibility with existing software, a special system-generated interrupt handles the execution of tasks previously performed during the vertical retrace Interrupt.' i.e., there is no *REAL* vertical retrace interrupt, it is only emulated. Now, from *DRAFT* Macintosh Family Hardware Reference: VIA Register B Bit Name Description 6 vH4 1 = SCSI IRQ Mask disabled and there is no horizontal blanking bit anywhere. Moral: when Apple says don't access the hardware, they mean it. I wish I could be more help. -- Joel West {ucbvax,ihnp4}!sdcsvax!jww (ihnp4!gould9!joel once I fix news) jww@sdcsvax.ucsd.edu if you must