Re: Antique computers (was Re: LILO and Zone Alarm)

[Rick Stevens <rstevens vitalstream com>]

Harold Hallikainen wrote:
> The first community college class I taught was assembly language
> programming on the PDP-8. We used the Teletype ASR-33 and punched paper
> tape. You'd use the toggle switches to load in one loader (the RIM loader,
> maybe), then read a paper tape that would load another loader (the BIN
> loader, I think). From there, you could load your editor and assembler
> from paper tape. Once you edited your source (on the Teletype), you'd
> generate a punched tape. Start the assembler and feed the source tape
> through twice (two pass assembler). Out would come an object tape. Start
> up the BIN loader again to read the object tape into core. Then debug with
> the front panel switches and lights.
>
> Remember how the PDP-8 did subroutines without a stack? The return address
> was stored in the "first" location of the subroutine. A return was then an
> indirect jump back to the first location, which would take you back to the
> one that called it. No recursion here!

That was common at the time.  The HP 2100 and 21MX used the same
mechanism as did the Xerox Sigma series.  The first instruction in each
subroutine was a "NOP".

"To understand recursion, you must first understand recursion."

> Remember the assembly instruction for two's complement (or negate)? It was
> CIA for Complement and Increment Accumulator. I used this just a month ago
> in teaching a logic circuits class on how two's complement arithmetic
> works.

Yup.  A common test for 8080/8085/8088 assembly programmers was "zero
the accumulator in a minimum number of cycles".  Most neophytes used
"MOV A,0" (two bytes, 12 cycles).  The correct answer was "XOR A" (XOR
accumulator with itself, 1 byte, 4 cycles).  Used to trip up a lot of
people.
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- Rick Stevens, Senior Systems Engineer     rstevens vitalstream com -
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