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From: wambacq@esat.kuleuven.ac.be
Newsgroups: comp.sys.mac.misc,comp.sys.mac.system
Subject: mac IIsi slowdown (long)
Message-ID: <1991Jan31.123728.4540@esat.kuleuven.ac.be>
Date: 31 Jan 91 12:37:26 GMT
Organization: Katholieke Universiteit Leuven,ESAT - dep. Elect. Engineering, Belgium
Lines: 49

Hi everyone,

some people on the net already reported the slowdown of the IIsi when using
colors instead of black and white. I think there is a very logical explanation
for this. The fact is that the video circuitry uses part of the IIsi's normal
memory for storing the image that is read out to the monitor. Whenever the
video circuitry accesses the memory, the CPU has to wait when it wants to access
the same memory bank. Now, when you use 8 bits video (color or greyscale, no
difference), instead of 1 bit video (black and white), the amount of reserved
memory for the video is much larger and the accesses are more frequent.
I did some tests with different depths (1, 2, 4 and 8 bits), and there is a
gradual slowdown when going from 1 to 2 and to 4 bits. The difference between
using 4 and 8 bits is larger.

There are however tricks to circumvent this slowdown. Therefore you have to know
that in the IIsi, memory is divided in two banks : bank A contains 1Mbyte and is
soldered on the motherboard, bank B consists of the SIMMs and varies in size (in
most cases 4Mbytes). There runs a bus between the CPU and bank B, but this bus
is not brought directly to bank A. There is a buffer between, which makes that
the CPU can access bank B at the same time that the video circuitry accesses
bank A. There would be no slowdown when the CPU would only use bank B. I did the
following test: I installed a RAM disk of 1 Meg, which sets asides a portion of
the memory. This turns out to be mainly bank A memory. Speed tests then show no
slowdown, no matter the number of bits, and this because bank A is not used by
the CPU as long as the RAM disk is not accessed. Disk access is however still
slowed down, but I think this is because I have no exact control over the use of
bank A and most probably some parts of it are still used by the system.

Now, this brings me to my question: would it not be possible to have an
init that makes part of the memory (in this case bank A) unavailable to the
CPU, by juggling with the tables that the MMU maintains? Of course, the
drawback would be that less memory is available to applications, but this is
in many cases not problematic. Is there maybe something like this already?

I also have another question: Apple's IIsi developer's note mentions that the
IIsi is designed to provide future support for network booting. Using the
control panel, one would set a flag in the ADB microcontroller that causes the
system to start up over a communications network such as Ethernet or LocalTalk.
Has anyone more information on this topic? Can this feature be used now
with DEC's PathWorks for the MAC?

Thanks,
Patrick Wambacq
Katholieke Universiteit Leuven
ESAT-MI2
Kard. Mercierlaan 94
3001 Heverlee
Belgium
wambacq@esat.kuleuven.ac.be