The GPU

The CorvusPrudensUnit features a graphics processing unit for offloading drawing from the processor. It’s a bit of a stretch to call it a GPU, but it does allow for much faster and more sophisticated monochrome drawing.

The GPU is accessed primarily from the and spt instruction, as gpu is one of the three memories that can be accessed by indirect addressing. GPU memory cannot be read – only written.

Frame buffer

0x0000-0x01FF

This range provides direct access to the display buffer. The buffer is organized into 4 rows of 128, 16-bit words. Each word corresponds to 16 vertical pixels, with bit position 0 being the top and bit position 15 the bottom. Since GPU memory cannot be read, it is recommended to use a frame buffer in ram and transfer it to the gpu frame buffer all at once.

Request buffer

0x0200-0x03FF

This range provides access to the GPU request buffer. Each request is two words long, with the first word specifying the parameters and sprite index, and the second word specifying the position. The screen position starts at 16, 16 – this facilitates easier partial drawing.

Request word 1	IIII IIII WWWW TVHC	(I) Sprite index, (W) sprite width, (T) text (1) or sprite (0), (V) vertical flip, (H) horizontal flip, (C) black (1) or white (0)
Request word 2	YYYY YYYY XXXX XXXX	(X) x position (Y) y position

Sprite buffer

0x0400-0x07FF

This range provides access to the sprite buffer. The graphics acceleration process reads the requests and copies the specified sprites to the screen buffer. Sprites can have an arbitrary width up to the buffer size of 256 words, but can only be a maximum height of 16 pixels.

Character buffer

0x0800-0x0FFF

This range provides access to the character “ROM”. The default font is loaded in during configuration, but can be rewritten if desired. The buffer consists of 1024 8-bit words, corresponding to a maximum text size of 4x8.

Clear address

0x1000-0x1001

Address 0x1000 determines whether the display buffer is cleared before every draw, and address 0x1001 is the word written for clearing. Naturally, this would typically be zero but you may occasionally want different patterns.