[Super Firey Dave]

I've recently started learning Assembly language and although it seems awfully tedious I'm making a bit of headway. I'm currently using a 6800 emulator found here http://www.hvrsoftware.com/6800emu.htm



I've made two subroutines charout and strout to enable output and everythings dandy.

My problem lies in that I'd like to be able to directly specify an x and y value for the string/char to be outputted on the 54x20 display.

The string buffer is at $FB00 and basically what I want to do is this:

output adjustment = (y * 54) + x
absolute output position = $FB00 + output adjustment

but I can't figure out how to do that

Hope that all made sense, and any help is greatly appreciated.

*EDIT*

Oh, I forgot to mention that x is a value between 0 and 53, and y is a value between 0 and 29

*END EDIT*

[job0107]

If your screen resolution gave you 80 bytes wide then you would use:
output adjustment = (y * 80) + x
absolute output position = $FB00 + output adjustment

And if your screen resolution gave you 160 bytes wide then you would use:
output adjustment = (y * 160) + x
absolute output position = $FB00 + output adjustment

You see the problem you are having is you are only looking at the matrix you have created and not the whole screen. If you remember the screen is one continuous string of memory locations. Therefore you have to count every memory location.

[Super Firey Dave]

I'm not sure if you've misunderstood me.
It's an emulator, it has a fixed resolution of 54 by 20.

In either case, regardless of what it's doing how does one write the code to multiple something by 54 times?

I understand how to double something using a shift left but multiplying higher eludes me.

[mab]

You need to use a combination of shift (*2) and addition (or subtraction.) You could use straight addition (add 54 times), but this would be the slowest.

One quick possibility, shift 6 places (assuming I counted correctly) to multiply by 64, saving the x2 and the x8 values so that you can subtract them from the x64 value -

Z x 54 = Z x (64-10) = Z x (64-2-8) = Z x 64 - Z x 2 - Z x 8

[Super Firey Dave]

Oh thanks, that's a really good idea.
I hadn't thought of keeping the eight times and the two times values.

My second question is do I actually calculate it given the accumulators can only hold two hex values each making the largest number 225?
Obviously even a small y value such as 5 multiplied by 54 would go over this limit.

[mab]

Thats where the carry and overflow bits come in.