[A83] LCD update freq and working with 16bit numbers on the Z80


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[A83] LCD update freq and working with 16bit numbers on the Z80




I'm just starting out programming TI83 assembly. I have programmed some 
8-bit assembler (6502/6510, the good old C64!) like 10 years ago, but I've 
no experience when it comes to Z80.
Anyway, I'm mainly doing graphical (demo style) effects at the moment, and I 
often have the need to add and subtract a 16 bit reg (hl) that I have 
pointing somewhere in the PLOTSSCREEN by 12 (one row).
Adding by 12 is easy, since there is a 16 bit add, but subtracting by 12 
seems to be very complicated, the only solution I could have come up with 
this far is this (which looks like it's waisting many clock cycles) :

ld a,l
sub 12
ld l,a
ld a,h
sbc a,0
ld h,a

I guess I'm overlooking something, or is this the best way to do it?

My second question - does anyone know how many times per second you can 
update the LCD using the _GRBUFCPY_V call? I mean, what 'FPS' is the 
theoretical max, assuming that nothing else but the buffer copy is done?

/Andreas

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