Am 13.06.2010 16:23, schrieb Orchid XP v8:

>> - 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.

> 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 ;-)

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.

If you want to go for classic 5V logic and thru-hole mounting (as 
opposed to SMD), maybe you won't find a suitably "sized" FPGA to fit 
your whole CPU inside, but you should surely find some PLCC-packaged 5V 
or 3V-5V FPGA to fit at least the complete control matrix; I'd suggest 
the Lattice ispMACH 4A5-64/32 44-pin PLCC for the job, which will give 
you up to 32 I/O pins with optional in- and/or output flip-flops, 2 more 
input-only pins, plus 32 bits worth of flip-flops for internal registers.

That would leave you with one 74xx for each register and each of the ALU 
functionalities to wire up, which should lessen the pain already. 
Alternatively, you could use another ispMACH for the ALU (which might 
also include the X, Y and Z registers), and (if required) a third one 
for all the rest.

Or, if you know how to solder SMD packages (or know someone to do that 
for you), you could go for something bigger, such as an ispMACH 
4A5-128/64 or 4A5-192/96, which should allow you to do all in one chip 
(except the main memory).

(The ispMACH 4 series is officially discontinued - which is a pity 
because I think the M4A5-64/32 was ideal for experimenting given that it 
runs on 5V and fits in inexpensive thru-hole mounted sockets - but some 
devices, including the M4A5-64/32 PLCC, are still available even in 
stock at Lattice, at prices of about 5$ apiece.)

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