glb_gtz's blog

DHGR

I have sucessfully emulated the Apple IIE text mode and I have coded the GR and HGR. This was the easy part. I am pretty sure Woz did not design the DHGR mode. I believe the Devil had a big hand in it's development. What I wanted to do was add a monochrome mode where I could get 560x192 mode to emulate a monochrome monitor. I also wanted close to the true individual dot color. On the Apple IIE emulator, when you write a 85 into location 8192, it displays differently depending on which monitor you setup. On a monochrome monitor, it appears as 4 monochrome dots. But on a color monitor, it appers as a solid gray line. This conforms to the way it is displayed on a composite monitor. Also, it conforms to Tech Note #3. But this tech note was written before high resolution VGA monitors were on everyones desk.

Major Issue

PROBLEM:
I was hoping to be able to run the system at 16.67 MHz just like the OSI system. But just like the original Apple ][, the video has to read the memory interleaved with the processor. The 16.67 MHz, is 50 MHz / 3, with two 50 MHz cycles for the processor and one cycle for the memory. Preliminary testing of the APPLE ][ text video is not stable at 16.67 MHz. Even at 12.5 MHz (50 /4), the pixel clock and the CPU clock have to be syncronized.

A new beginning

I plan on using this blog to document my newest project. I found with the last project, documentation after the fact is more difficult than I thought it would be. I have a hard time remembering why I did some things the way that I did.

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