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  From: Richard Sharpe <sharpe@ns.aus.com>
  To  : David Newall <davidn@rebel.net.au>, Mike Andrew <davidn@rebel.net.au>
  Date: Thu, 29 Jun 2000 21:25:30 +0900

Re: More space required

I think David has lost his marbles :-)

At 06:04 PM 6/29/00 +0930, David Newall wrote:
>Hmmm, interesting perspective.  It used a 16 bit CPU with an 8 bit data bus
>(8088), and came standard with 128KB of RAM.  Options included upgrades to
>512MB of RAM but no further.  It included the ubiquituous cassette

512MB of RAM? In 1981?

>interface, and a choice of one or two floppy disk drives.  Video display
>was provided by a choice of MDA (monochrome display adaptor) or CGA (colour
>graphics adaptor)  System clock ran at ... goodness me, I'm scratching to
>recall whether it was 5MHz or 8MHz.  Maximum address space was 1MB, of
>which the top 384MB is reserved for BIOS and hardware, leaving 640KB

That's an awfully big BIOS, although I admit, the PC required lots of
papering over.

>addressable for programs and data.
>
>The PC included an 8 level programmable interrupt controller, of which I
>think five interrupts were unassigned.  This was a good thing and a bad
>thing.  Without doubt interrupt processing is simplified if you know for a
>fact that only one device uses any interrupt level.  On the other hand, we
>continue with the legacy of IRQ conflicts to this day.
>
>It's competitors at the time were the Apple ][, which used a RISCish 8 bit
>CPU (6502), and came (at the time) standard with 48MB RAM.  The Apple ][

That was hard to do with only 16 address bits. Hmmm, 25 address bits
required there.

>was venerable even then, and also included a cassette interface.  It came
>standard with colour graphics, had a choice of between zero and twelve
>floppy disks, and between zero and six hard disk drives.  The Apple ][
>system clock ran at 1MHz.  The Apple ][ had two interrupts: One maskable
>(ie the hardware could choose to ignore it) and one non-maskable (it could
>not be ignored.)  Thus the Apple ][ never did have a problem with interrupt
>conflicts.  Maximum address space was 64KB, of which 12KB was preloaded
>with BIOS, 4KB was reserved for general hardware, and various amounts,
>depending on video mode, was reserved for the screen.  The fractured
>address space was one of the ]['s biggest weaknesses.
>
>Another competitor was the Apple ///, which initally came standard with
>128MB RAM, and later 256MB.  The Apple /// could be upgraded to 512MB.  It

They would have been very expensive back in the early '80s. No wonder they
wern't so successful :-)

>also had colour graphics as standard.  The Apple /// was, in fact, a
>typical Apple hack, which is to say it abused the intended design in quite
>clever ways.  The Apple /// system clock ran at 2MHz, and, like the IBM PC,
>included a standard real time clock.  The Apple /// was an upgraded Apple
>][.  It had a maximum address space of 64MB of RAM and used sophisticated
>bank-switching to take advantage of the addition RAM that the machine could
>accomodate.

Hmmm, well the Telenet packet switches in the early 80's were based on the
6502, and they used bank switching as well, so we could have 1MB of memory
available. It would have been wonderful having hardware that allowed you to
bank switch 1MB chunks in a 64MB memory space :-)


Regards
-------
Richard Sharpe, sharpe@ns.aus.com
Samba (Team member, www.samba.org), Ethereal (Team member, www.zing.org)
Co-author, SAMS Teach Yourself Samba in 24 Hours
Author: First Australian 5-day, intensive, hands-on Linux SysAdmin course
Author: First Australian 2-day, intensive, hands-on Samba course

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