[TUHS] Happy birthday, PDP-8!

Johnny Billquist bqt at update.uu.se
Tue Mar 28 09:49:22 AEST 2017


And, as usual I have to correct myself a little, but I'll also tie in a 
bit with (almost) Unix in doing so. :-)

On 2017-03-28 01:33, Steve Nickolas wrote:
> On Tue, 28 Mar 2017, Johnny Billquist wrote:
>

[...]

>> The routine:
>>
>> COUNT,  0
>>        CIA
>>        DCA    CHR        / Save the negative of the word to search for.
>>        CMA
>>        TAD I  COUNT
>>        DCA    PTR        / Setup pointer to the address before the
>> buffer.
>>        ISZ    COUNT      / Point to next argument.
>>        TAD I  COUNT
>>        CIA
>>        DCA    CNT        / Save negative value of size.
>>        DCA    RESULT     / Clear out result counter.
>> LOOP,   TAD I  PTR        / Get next word in buffer.
>>        TAD    CHR        / Compare to searched for word.
>>        SNA               / Skip if they are not equal.

I knew I felt that something wasn't right, but it wasn't until I looked 
at your reply, Steve, that I found what it was.
This SNA should have been SNA CLA...

>>        ISZ    RESULT     / Equal. Increment result counter.
>>        ISZ    CNT        / Increment loop counter.
>>        JMP    LOOP       / Repeat unless end of buffer.
>>        CLA               / All done. Get result.

and CLA should not have been there.

>>        TAD    RESULT
>>        JMP I  COUNT      / Done.
>>
>> PTR=10
>> CNT=20
>> CHR=21
>> RESULT=22
>>
>>
>> Addresses 10-17 are the index registers, so the TAD I PTR instruction
>> will autoincrement the pointer everytime, and the increment happens
>> before the defer, which is why the initial value should be one less
>> than the buffer pointer.
>>
>> Hopefully this gives enough of an idea, but unless you know the PDP-8
>> well, you might be a little confused by the mnemonics.
>>
>> As you can see, the return address at the start is used for more than
>> just doing a return. It's also your argument pointer.
>>
>>  Johnny
>>
>>
>
> Actually, that reminds me of ProDOS-8 on the Apple ][, which uses a
> similar mechanism to pass parameters.

Interesting. Since the Apple ][ used the 6502, I would have assumed it 
used the stack, but I've never used the 6502 much, so I don't know if it 
possibly can make sense to write code this way on that processor.

Speaking of which, when the PDP-11 was introduced, it wasn't obvious 
that this style of coding wasn't sometimes appropriate, so the PDP-11 
have the means of doing this as well.
If anyone ever wondered about the strangeness of the JSR instruction of 
the PDP-11, it is precisely because of this.
What it does is that it pushes the argument register on the stack, loads 
the PC into the argument register, and then sets the new PC.
So, if you use some other register than R7 as the argument to JSR, that 
register then points to the word following the JSR, where you can have 
your arguments. And you fetch them with indirect autoincrement, and at 
the end you do an RTS with that same register, which at that point 
points beyond the arguments you had. And the register used for the 
argument pointer is restored by the RTS.

I've used it once or twice on a PDP-11, but this whole scheme only works 
if I-space and D-space are the same, so there are limitations.

I doubt Unix ever used this, but maybe someone know of some obscure 
inner kernel code that do. :-)

	Johnny



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