4 ms·
The IIe, IIc, and IIgs had a means to track the vertical blank signal, but the original II did not. So a hack was discovered to let the 6502 see the video data
by joezydeco 5y ago
The IIe, IIc, and IIgs had a means to track the vertical blank signal, but the original II did not.
So a hack was discovered to let the 6502 see the video data being clocked out. It's described in great detail in Don Lancaster's book.
https://www.tinaja.com/ebooks/enhance_vII.pdf https://www.tinaja.com/ebooks/enhance_vII.pdf
The gist is that the last byte of each horizontal raster is not displayed on-screen, but with the hack the 6502 can see it. So if you set the low bit of the last byte to 1 for every horizontal line except the last one, now you know when the hardware has sent out the last byte of the last line. Vertical Blank comes right after that.
- anyfoo 5y agoAh, thanks. Like the poster you responded to probably, the actual bit (hah) that I was missing was that the last bit was apparently no used by the framebuffer in any way, i.e. it's available for synchronization because changes to it are not visible on the screen.