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Path: utzoo!watmath!clyde!bonnie!akgua!whuxlm!harpo!decvax!mcnc!philabs!cmcl2!seismo!umcp-cs!cvl!umd5!zben
From: zben@umd5.UUCP
Newsgroups: net.micro,net.micro.apple
Subject: Re: head noise in apple IIe
Message-ID: <573@umd5.UUCP>
Date: Wed, 12-Jun-85 11:02:29 EDT
Article-I.D.: umd5.573
Posted: Wed Jun 12 11:02:29 1985
Date-Received: Sat, 15-Jun-85 09:54:00 EDT
References: <740@burl.UUCP>
Reply-To: zben@umd5.UUCP (Ben Cranston)
Organization: U of Md, CSC, College Park, Md
Lines: 61
Xref: watmath net.micro:10781 net.micro.apple:1975
Summary: Here is the information (again, sigh)

Well, this has been posted before, and I'm *sure* it will be posted again.
Nevertheless, here's my stab at it.

The display uses only *SOME* of the memory assigned to the hardware display.
The few bytes that it does *NOT* use are assigned to the peripheral cards for
use as RAM memory.  Some of the peripheral drivers (e.g. the disk driver)
saves *state information* in these locations, and if you blow away these
bytes you will confuse the driver, causing the problem you describe.

The relevant documentation can be found on page 125 of the Apple ][e
reference manual, the section entitled: "Peripheral-card RAM Space".
My boss has the office ][+ and all its books at home right now, but this
information *IS* in the ][+ reference manual.

To avoid blowing away there RAM locations, you will have to stop saveing and
restoring display memory with a block IO from 400 to 7FF or 800 to BFF and
instead do a bunch of little IOs.  First cut is to do 24 of them:

     400-427
     480-4A7
     500-527
     580-5A7
     600-627
     680-6A7
     700-727
     780-7A7
     428-44F
     4A8-4CF
     528-54F
     5A8-5CF
     628-64F
     6A8-6CF
     728-74F
     7A8-7CF
     450-477
     4D0-4F7
     550-577
     5D0-5F7
     650-677
     6D0-6F7
     750-777
     7D0-7F7

But, note that *the order doesn't matter*, as long as you read them in with
the same order you wrote them out!  Some of the ranges are contiguous, and
thus the task can be simplified to 8 IO requests:

     400-477  (rows  1,  9, 17)
     480-4F7  (rows  2, 10, 18)
     500-577  (rows  3, 11, 19)
     580-5F7  (rows  4, 12, 20)
     600-677  (rows  5, 13, 21)
     680-6F7  (rows  6, 14, 22)
     700-777  (rows  7, 15, 23)
     780-7F7  (rows  8, 16, 24)

If you compare this against the table on page 125, you will see that this
IO sequence avoids the Peripheral card RAM locations...

-- 
Ben Cranston  ...{seismo!umcp-cs,ihnp4!rlgvax}!cvl!umd5!zben  zben@umd2.ARPA