GIME Horizontal Retrace Register Update Table

This is a proposal to the people who make FPGA versions of the CoCo 3′s GIME chip.

I was recently reading about the Amiga’s Copper chip. I thought something similar could be added to a GIME implementation. The following is my idea, for what it’s worth.

Addresses used:
FF99, Bit 7 Horizontal retrace update enable
FFEA-FFEC Address (24 bit) table start
Table format:
0 byte Scan line
1 byte GIME register number
2 byte New value


If enabled, during the vertical retrace, the 24 bit table start address is loaded. Then the first byte of the table is deference. If the scan line value (table byte 0) is $FF, then the new value is written to the register number. The table pointer is advanced (by three bytes) and writes are repeated until the scan line is no longer $FF. This allows you to set any video registers before they are latched for the frame.

During every Horizontal retrace the scan line declared in the table is compared to the current scan line. If there is a match, the table value is written to the specified GIME register. Then the next table entry scan line is loaded.

If the scan line does not match, the process is paused until then next horizontal retrace.

If there is time, multiple GIME registers can be changed during horizontal retrace.

Register number zero refers to $FF90 (Initialization register #0). It goes up from there.

Register numbers $FE will trigger an FIRQ or IRQ. Nothing will be written to an internal register. Bit 6 will determine which interrupt is generated ($FF92 or $FF93).

Register number $FD will cause the GIME to reload the video start address from $FF9D and $FF9E. Nothing will be written to an internal register.

Example Programs

I implemented the above in a private MAME branch. I wrote some demos and made these screen captures. All this code can be found here.


In BASIC, switch to HSCREEN 2 and draw a grid of 64 circles. Eight every row, for 8 rows. Use palette index 1 to 8 for every row. Load in an assembly language program to install a horizontal sync table to update the palette color to show all 64 colors on the screen at the same time.



In BASIC, switch to HSCREEN 2, then load a binary file. This file contains:
1. Image data to be loaded in the Hi-res screen memory.
2. The first 52 lines are 128 bytes wide
3. The rest are 256 bytes wide.
4. Hsync table to switch to 256 byte wide video at line 52.
5. Code to install hsync table and turn it on.
Then the BASIC program will update the horizontal scroll register in the table to match the horizontal joystick position.



In BASIC switch to HSCREEN 2, Draw random circles. Load hsync table and enable it. Near the bottom of the screen the GIME is instructed to switch to text mode and change the display address to be the bottom of the 40 column text screen.
Repeat drawing circle to the graphics screen and printing text to the 40 column screen.



In BASIC load binary from disk, wait for disk to stop spinning. Switch to HSCREEN 2 and have BASIC draw random circles. Setup horizontal table to trigger IRQ interrupt on scan line 100.

Interrupt service routine quickly sets all palette registers to zero. Then immediately restore RGB palette values. Repeat on next frame.