Hi,

Docopy by grauw:

There is a version faster version of the routine docopy which does want the
user to set status register 2 at all times.
This I do on the Interrupt.
Docopy is in my backgroundscroll routine, which is not on the interrupt.

However I do not seem to be able to set status register 2.

I tried several things, but they don't work.

Can somebody give me an example of how to set St.R#2 on each interrupt?

VDP registers can be written but not read.
VDP status registers can be read but not written.

he doesn't mean setting the value of S#2, but selecting S#2.

anyways, here's how to do it:

ld a,2
out ($99),a
ld a,15+128
out ($99),a

Quote:

VDP registers can be written but not read. VDP status registers can be read but not written.

hmm, maybe I misread something.
Docopy is a routine that writes 15 bytes starting at register 32.
Status registers 2 and 0 are used (to check if the vdp is done copying)

However, the faster version of Docopy only requires the usage of
status register 0.... on 1 condition:

That status register 2 is allways set (or never reset???)

I dont quite get it.....

Plus, this setting of status register 2 is supposed to be done in the
interrupt handler.

I tried something like writing the value 2 to the register which is
used to read status registers... and then using in a,($98)

but this doesnt seem to do the trick

What is means is, if you know for sure what status register is selected for read in R#15, you don't have to set it again. However, you do need to be certain, which means you code (including interrupt routines) must run preditable.

If you use custom interrupt handler and you have disabled V-blanc interrupts, you don't have to switch back to status register 0.

This is because MSX-BIOS interrupt handler does not change back to status register 0 if you change it and you can't exit V-blanc interrupt without reading status register 0.