Memory model

The MB8 VM exposes a single 64 KiB address space. All reads and writes go through the bus (crates/mb8/src/dev/bus.rs), which forwards them to RAM, ROM, or an MMIO device based on the address range.

Layout

The bus rejects the reserved regions with unimplemented!().

Bus

• CPU memory accesses always call into the bus, which in turn calls the matching device read/write.
• Devices own their buffers; the bus itself does not store data.

RAM (crates/mb8/src/dev/ram.rs)

• Plain byte-addressable memory. Writes update the backing array; reads return what was last written.
• RAM_SIZE = 0xC000. The stack grows downward (STACK_TOP = 0xBFFF, STACK_BOTTOM = 0xBF00).

ROM (crates/mb8/src/dev/rom.rs)

• Backing store for program code (ROM_SIZE = 0x1000).
• The device currently accepts writes from the bus, but programs should not rely on mutating ROM; this may be blocked in the future. ROM is meant to hold the kernel/boot image.

GPU (crates/mb8/src/dev/gpu.rs)

• Registers live at 0xF000 (offsets relative to that base):
  • 0x0000 — mode register. 0x00 = off, 0x01 = TTY.
  • 0x0001 — TTY data register. When mode is TTY, each write pushes a character to the screen and advances the cursor.
• Reading 0x0000 returns the current mode. Other reads are currently unimplemented.

Keyboard (crates/mb8/src/dev/keyboard.rs)

• Registers at 0xF100 (offsets relative to that base):
  • 0x00 — STATUS. Returns 1 when keys are queued, otherwise 0.
  • 0x01 — DATA. Reading pops the next key code from the queue; returns 0 when empty.
• Writes are ignored.

Disk (crates/mb8/src/dev/disk.rs)

• Registers at 0xF200 (offsets relative to that base):
  • 0x0000 — BLOCK number to operate on.
  • 0x0001 — CMD (0x00 no-op, 0x01 read, 0x02 write).
  • 0x0002–0x0102 — 256-byte disk buffer used for reads/writes.
• CMD operations move data between the internal image and the buffer; buffer reads/writes go directly to the 256-byte window.