MKrieg@UMASS.BITNET.UUCP (02/25/87)
/\/\ark <riegsman -=*=-- 561 Hampshire College -=*=- Amherst, MA -=*=- 01002
I've been tempted several times in the last few days to respond to "things"
that have been posted here, but have so far refrained. But, ...
RE: Disk conversion...
The original disk ][ controller ( the one that runs your old-style 5
1/4" drive(s)) has almost no smarts of its own. You can literally
intercept the data coming from the disk on a bit-by-bit basis. While
this makes it seem like you might be able to do anything you want
with an Apple and a 5 1/4" drive, 1) the processor is not -quite-
fast enough to use the data as it comes in...the best you can do is
buffer it for later and 2) the disk controller, because of it's lack
of brains, can "get lost" on any given disk track if the data is not
formatted to prevent it. Specifically, no more than two bits in a
row may be zero. (Under the 13 sector controller [remember them!] no
more than one (!) bit could be zero in a row.) The disk driver
program changes data from a happy 8-useful-bits-per-byte to a
somewhatskewed 6-useful-bits-per-byte format before it writes it out,
and de-converts when it reads it back in.
You cann write your own disk driver, but be very careful about
timing. (this is how every single program ever released for the Apple
up until about three years ago had its own disk format...hence the
need for "bit-copiers") If you try this, one final word of advice:
PUT THE HEAD BACK INTO THE READ MODE BEFORE TURNING OFF THE DRIVE
MOTOR!!! Geoffrey Engelstein and I (ever hear of a game called
"Starblaster" from Piccadilly Software?...we wrote it.) spend about
45 straight hours trying to find that one.
RE: Relative Clock Speeds
THe 6502 is a "pipelined" processor; each part of it can do something
at once. The memory acces part can be fetching the next piece of
data *while* the Arithmetic Logic Unit is performing an "add"
operation.
A 1Mhz 6502 is -generally- (no flames, PLEASE!) about as fast as
3-4Mhz Z-80, because the Z-80 can only to one thing at a time, ie,
fetch an instruction, fetch a data value, add, deposit the value,
etc..
We (Apple folks) are blessed (!??) with the hand-coded magic of one
Mr. Steve Wozniak. This man seems to have some kind of obscure
mutation that allows him to write faster, smaller, more efficient
code than anyone else. So beyond pure processor /speed/ we have the
work of a brilliant man lurking in each machine.
RE: High-res character Generators...
Well, it seems that this kind of thing is in high demand. The //gs
has a *really* *nice* on in ROM, but that's quite a large investment
to make for a piece of utility software.
If anyone is interested, I would be willing to post a reasonably
robust and complete HRCG (Shareware...$5) to the net, DOES ANYONE
WANT THIS? (write to me, so if no one else does, I can send it just
to you.) I could also post a couple of fonts I've got, again, only if
people want this.
I've been working with Apples (doing assembly language development work) since
about '79, so if anyone has any weird assy lang questions, I'd be happy to try
and help.
,-Mark-Kriegsman--(see-diagram-2)---------------.
Claimer:
1) you can only act once. BITNET: mkrieg@umass
2) einmal ist keinmal. CSNET: mkrieg%hamp@CS.UMass.EDU
'. nothing /really/ matters. INTERNET: mkrieg%umass.bitnet@wiscvm.wisc.edu
UUCP: ...seismo!UMASS.BITNET!mkrieg
But don't let that stop you, USMail: Hampshire College, Amherst, MA 01002
happiness is subjective. Phone: (413) 549-5113 <odd hours, but try!>
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