The Atto-8 microcomputer is a minimalist computer based on the Atto-8 microprocessor, as specified in /spec/microprocessor.md. It equips the processor with a clock, memory, standard input/output, a display, and a pair of D-pad controllers. It is designed to be a simple system that takes full advantage of the Atto-8 microprocessor. It is intended to be used as a learning tool for students and hobbyists, and as a basis for more complex computers.
- 1 MHz clock
- 256 bytes of memory
- Standard input/output
- 16×16 pixel display
- Two D-pad controllers
The Atto-8 microcomputer supports standard input/output through the input buffer, a byte located at address 0x00. Standard input and standard output are buffered in hardware; reading from the input buffer will return the next byte from stdin, and writing to the input buffer will send the byte to stdout. If stdin is empty, reading from the input buffer will fall back to returning controller states. Reads and writes to to address 0x00 are intercepted by the microcomputer, and, therefore, the input buffer does not behave like other memory regions.
On startup, stdin is initialized with the contents of memory address 0x00. That is, the first read from address 0x00 will return the byte located at address 0x00, and subsequent reads will return either bytes from standard input or controller states.
The Atto-8 microcomputer is equipped with a 16×16 pixel monochrome display. It fetches rows of pixels from addresses 0xE0..0x100, the display buffer, and displays them from left to right, top to bottom. Refer to the following diagram, in which [ 0xXX ] represents a row of 8 pixels fetched from address 0xXX:
[ 0xE0 ] [ 0xE1 ]
[ 0xE2 ] [ 0xE3 ]
[ 0xE4 ] [ 0xE5 ]
[ 0xE6 ] [ 0xE7 ]
[ 0xE8 ] [ 0xE9 ]
[ 0xEA ] [ 0xEB ]
[ 0xEC ] [ 0xED ]
[ 0xEE ] [ 0xEF ]
[ 0xF0 ] [ 0xF1 ]
[ 0xF2 ] [ 0xF3 ]
[ 0xF4 ] [ 0xF5 ]
[ 0xF6 ] [ 0xF7 ]
[ 0xF8 ] [ 0xF9 ]
[ 0xFA ] [ 0xFB ]
[ 0xFC ] [ 0xFD ]
[ 0xFE ] [ 0xFF ]
The display buffer behaves as any other memory region and can therefore both be read from and written to by a program.
The Atto-8 microcomputer is equipped with a pair of memory-mapped 4-button D-pad controllers. If stdin is empty, reading from the input buffer, a byte located at address 0x00, will fall back to returning controller states. In that event, the lower 4 bits of the input buffer represent the state of the buttons on the primary controller, and its upper 4 bits represent the state of the buttons on the secondary controller. It is bit-mapped as follows, where 0 represents the least significant bit:
7 6 5 4 3 2 1 0
| | | | | | | |
r l d u R L D U
| | | | | | | |
| | | | | | | +--- Primary Up
| | | | | | +----- Primary Down
| | | | | +------- Primary Left
| | | | +--------- Primary Right
| | | +----------- Secondary Up
| | +------------- Secondary Down
| +--------------- Secondary Left
+----------------- Secondary Right
Primary Secondary
U u
L + R l + r
D d