VERA Line IRQ

Elektron72 · Post by **Elektron72** » Mon Feb 08, 2021 3:21 pm

Looking at the VERA Programmer's Reference, it appears that the line at which the raster interrupt occurs is specified by the contents of VERA register $9F28 and one additional bit in register $9F26. However, given that the Commander X16 can have a maximum of 640 lines, shouldn't there be another bit to allow triggering an IRQ on lines past 511?

Guybrush · Post by **Guybrush** » Mon Feb 08, 2021 4:24 pm

VERA's native resolution is 640x480. That's 640 pixels horizontally by 480 pixels vertically. So, there are no 640 lines, only 480.

Of course, there's also the part where there's no image output, but according to http://www.tinyvga.com/vga-timing/640x480@60Hz, even then there's only 525 scanlines. Anyway, I believe we can ignore those extra 13 scanlines we are not able to set the IRQ for.

Elektron72 · Post by **Elektron72** » Mon Feb 08, 2021 4:40 pm

Thank you for reminding me that I somehow mixed up the vertical and horizontal resolution.