lr-vm/docs/memory.md

Memory

The vm addresses memory by segment and offset. There are 256 segments each with 256 bytes. Addresses in this documentation are denoted as segment.offset. For example, 64.2 refers to the third byte of the 65th segment. 0.0 refers to the first byte of the first segment. The bytes in memory are unsigned and store a value between 0 and 255 (inclusive).

Memory Map

• Segments 0 through 63 (inclusive) are general purpose RAM for your program.
• Segments 64 through 127 (inclusive) are read and written to by the graphics processing unit (GPU) and input processing unit (IPU).
• Segments 128 through 255 (inclusive) are for read only data that is loaded when the machine starts.

GPU and IPU Map

• 64.0 through 79.255 (inclusive) are for 256 sprite attributes. Each sprite attribute is 16 bytes long.
• 127.0 through 127.1 (inclusive) are the 16 bits indicating which joystick buttons are pressed. There are 8 buttons for each of the 2 joysticks.
• 127.16 is the background color of the screen stored as an index into the screen 256 color palette.
• 127.17 contains flags to change the state of the GPU and indicate the status of the GPU. (All 0 by default)
  • Bit 1: Toggled by the GPU when a screen draw is finished.
  • Bit 0 (LSB): Set to 1 to enable the GPU to render to the screen. Set to 0 to disable the GPU to render to the screen (leaves current image).