Sunday, 30 June 2013

A slightly less barmy answer

I had an idea last night. (Gives you some idea how dull my nights are).

The carry transfer method is a bit slow, the indexed jump method is a bit unwieldy. So why not combine the two ?

So, in the current implementation, the value to be written is halved and the LSB is stored in the carry, which gives us only 8 values to write out. So it's about twice the speed of the bitshifting one (14 cycles rather than 33) and about half as long again, but still fits comfortably in one page.

What it is doing is (in C) something like the code below. It does look very bizarre as a way of copying into memory but it works because we can preserve the carry flag when loading 'addr' into the RAM address register, but we can't preserve the accumulator.

It's a recurring nightmare writing this that there's a really obvious simple solution to this which I can't see...... and these posts will then make me look really dim.

// same code effectively as memory[addr] = acc
carry = acc % 2           // put LSB of acc in carry bit
acc = acc / 2             // rotate A right e.g. halve it.
switch (acc) {
   case 0:     memory[addr] = 0+carry;break;
   case 1:     memory[addr] = 2+carry;break;
   case 7:     memory[addr] = 14+carry;break;



No comments:

Post a Comment