Part 3 – Writing to the framebuffer

The previous instalment of this project saw a working VGA framebuffer being filled by an automated hardware process.  Since then I’ve managed to get the TG68 processor running a program from a blockram-based ROM, and writing to the DE1 board’s SDRAM.

The memory map for the TG68 looks something like this:

• $000000: ROM (at the moment the ROM code just runs sequentially from $000008.  Will eventually need to put an interrupt table in place and move the code elsewhere.)
• $0FFFFA: A couple of variables
• $100000: The frambuffer
• $7FFFFE: The initial stack pointer

For the ROM I use an M4K blockram, with initial contents specified with an MIF file.  To create the MIF I first assemble the code with Easy68K, then convert the S-Record file it produces to MIF format using srec_cat, like so:

srec_cat in.S68 -o out.mif -mif 16

The ROM program currently backfills the screen, then draws random rectangles in an incrementing colour.  The display is Hi-Colour (5-6-5) bit, which is dithered to fit the DE1’s 4-bit-per-gun output.

The complete project, for anyone who’s interested, can be found here.

Currently the fill-rate is pretty lousy since only a single 16-bit word can be written per slot.  I need to add a writethrough cache of some kind, preferably one capable of merging successive writes to a single burst.

Possible future plans for the project include:

• CPU cache – separate instruction and data.  Just one burst (64 bit) initially.
• Writeback cache – to allow (a) the CPU to continue working while a write is in progress, and (b) to gang sequential writes into a single burst write.
  Sprite controller – just one sprite for mouse pointer.
• Multiple screenmodes.
  •  8bit
  •  lowres
  •  scandoubled?
  •  PAL?
• Sprite controller.  Just one sprite, for a mouse pointer.
• A colourtable for indexed modes
• VBlank interrupt
• Interrupts in general
• SD card interface
• PS/2 controller
• Hardware registers for framebuffer address, sprite position, PS2 data, etc.
• Sound