>>> - As your commands are all 2 bytes wide, you can require that they be
>>> 16-bit aligned, so you can increase address space available for
>>> programs to 512 bytes (=256 instructions).
>>
>> In other words, make memory 16-bit addressible rather than 8-bit
>> addressible.
> 
> Well, you could do /that/, too, but I was merely thinking of making the 
> memory 9-bit addressable, by using a 9-bit program counter and 
> automatically shifting the value left by 1 bit (filling with zero) when 
> loading an 8-bit value into the PC.

Oh right, I see...

>> The other possibility, of course, is to just connect up more address
>> lines. Makes little or no difference in software, but in hardware I
>> suddenly have to put a hell of a lot more wires together. ;-)
> 
> Sure ;-)

I did briefly entertain the idea of a 4-bit design. Certainly my CPU 
loads 16 bits from RAM on every instruction cycle, yet has only 17 valid 
opcodes (not 2^16 or even 2^8). On the other hand, a 4-bit address space 
is about as useful as... um...

> You should really have a look at FPGAs if you want to save the money, 
> time and hassle of wiring up a hell lot of 74xx gates.

I have a sinking feeling that if I went to the [extreme] expensive of 
getting an FPGA rig, I'd probably then find that I'm too stupid to make 
it do anything anyway! :-/

-- 
http://blog.orphi.me.uk/
http://www.zazzle.com/MathematicalOrchid*

Post a reply to this message