Friday, 30 May 2014

Virtual CPU - Signed Addition Clean Up & ROM Discussed

In today's CPU post I want to just clean up the signed addition example, we've covered the electronics but I've had a couple of messages asking how I might integrate switching into the CPU.

Well, simply for our Virtual CPU I'm going to include the signed addition based on the signed mode flag... We already had this signed mode flag in the CPU, and we default the flag to false or "off".

So that is a simple "if" statement within the "Add" function.

To integrate the switching we invent two new OP Codes, one to switch into Signed processing and one to switch to Unsigned processing.

I'll leave you guys to think about how the programmer has to remember which mode they were in, and hence what the bit patterns they have represent.

I'm also going to leave multiplication whilst in Signed Mode as an exercise for you to address yourselves, if you want to mail me your solutions, I'll happily take a look (if I find a minute).

So our op codes now run from zero to twenty-seven.  So with 28 operations what could a machine do?

You might think not very much, but the real 4004 (though not yet the same instruction codes as our virtual code) operated with just 46 instructions total, more than our code at present, but still not a lot.  Intel have kindly published scanned copies of their original datasheets and so we can peek into the depths of their instruction set here:

Numerically we can see almost immediately their instructions 2 and 3, are about Fetching... Fetching Immediate and Fetching Indirect (from ROM)... What is Fetching?  Well, in other machines, many assemblers and in our Virtual CPU the concept of Fetching is called "Loading" and we Load0 and Load1.  Both those instructions load from memory into the CPU, this is "Immediate" it immediately moves a value from volatile storage into the processor.

Indirect for our CPU would actually be the main program loading a program from a file, the file is our ROM or non-volatile storage and we load it into the RAM to use it.  However, we don't fetch from ROM.

I had been asked to add a ROM to the Virtual CPU, however, all a ROM is is addressable memory which can't be changed, so if you wanted to create a ROM yourself you can, create a byte array in your program, load from a disk file, or just insert data into the array upon construction.

And then add two new Op Codes you want to fetch from the ROM.  You can then add the ROM to your CPU as a reference...

I hope this gives you some ideas and you go a head and try to write a small ROM.

Next on our agenda will be "Interupts"... Stay Tuned.

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