OT - Memory usage in days of yore

[Georg Wrede]

Janice Caron wrote:
> On 10/04/2008, Georg Wrede <georg at nospam.org> wrote:
> 
>> 16k ROM, and either 16k or 48k RAM, according to
>> http://en.wikipedia.org/wiki/Sinclair_ZX_spectrum
> 
> 
> That was the Spectrum, a different machine, and a couple of years
> later. We're talking ZX80 here.
> 
> Course, we didn't have the internet back in those days. Hell, we
> hardly had computers! But this link should shed some light.
> http://computermuseum.50megs.com/brands/zx80.htm

Ah, ok.

Well, I had a grand 3.5k on my VIC-20.

The biggest advantage to having a ridiculously "small" computer was, 
that one would learn it through and through. I remember knowing the 
memory map by heart, the locations of every system variable, and also 
the entire circuit diagram. Haven't done that with any of my later 
computers.

It also used the 6502 processor, which was more primitive than the ZX-80 
and it only hand the registers x, y, a, status, stack pointer, and 
program counter. When other processors had auxiliary registers, the 6502 
used 128 bytes of memory at the beginning of the address space. The 
stack was limited to the next 128 bytes.

Interestingly the stack (one-eighth of a k) was shared between the 
system, the interpreter, and user programs. At the time I was amazed at 
how things didn't get mixed up. Today I'm amazed that you can do 
anything with a stack that small. The size was not adjustable because 
the processor used special, fast instructions to address the first 256 
bytes of memory.


For this computer I wrote a multitasking process that visualized the 
contents of arbitrary memory areas during running of other programs, in 
real time. I wanted to do that for two reasons. First, it was a major 
challenge: there was no reason to expect that I could pull it off. 
Second, I wanted to see "what actually happened" inside the computer 
when it was running programs. I'm a visual kind of learner.

I used the program to study stack allocation, storage of Basic programs 
in the program area (and to learn how they were stored in compressed 
form -- you have to remember, at the time bookshops didn't carry 
computer literature, let alone something VIC-20 specific. Not to mention 
we didn't have WWW, or even modems), heap activity, and activity in 
system variables. Oh, man, who needs sleep or food with this stuff!


I didn't have an assembler, nor even a Monitor program. But then I 
didn't really mind. The assembler for 6502 was so easy to learn and use, 
that I did it all on pieces of scrap paper or backs of envelopes. And it 
was very easy to relocate the code, especially if you avoided using 
absolute addresses to within the code.

Oh, and I didn't have any books about it. I found a listing of the asm 
commands in an appendix, and that was that. I guess I'm lucky, a more 
complicated instruction set could have been too much.


Years later, when I already had a CP/M Office Computer (as opposed to a 
toy), the Kaypro-II, I found a mail-order bookshop in a german computer 
magazine, where I bought the complete and annotated listing of VIC-20 
firmware (bios and basic roms). I learned a lot from that too. Heh, and 
German computerese isn't that hard to understand. There's a certain 
feeling when you've read the entire rom listing. You sort of feel 
there's something you know through-and-through.