System Calls

System calls live at 0xE500 (kernel/syscalls.asm). To invoke one, load the call ID into R0 and CALL 0xE500. Inputs and outputs travel through the registers listed below; all other registers are caller-saved.

• 0x01 — SYS_GPU_MODE
  Input: R1 mode byte (0x00 off, 0x01 TTY). Writes the GPU mode register at 0xF000.

• 0x02 — SYS_WRITE
  Input: R1 character byte. Sends it to the GPU TTY data register at 0xF001.

• 0x03 — SYS_WRITELN
  Input: R1:R2 address of a zero-terminated string. Streams characters to the TTY data register until 0x00.

• 0x04 — SYS_WAIT_FOR_KEY
  Blocks until the keyboard status register (0xF100) is non-zero. No outputs.

• 0x05 — SYS_READ_KEY
  Output: R0 key code popped from the keyboard data register (0xF101). Returns 0 if the queue was empty.

• 0x06 — SYS_DISK_SET_BLOCK
  Input: R1 block index. Stores it in the disk block register at 0xF200 for later operations.

• 0x07 — SYS_DISK_READ_BLOCK
  Uses the previously selected block and copies it into the disk buffer window (0xF202–0xF302).

• 0x08 — SYS_DISK_WRITE_BLOCK
  Flushes the current disk buffer window into the previously selected block.

• 0x09 — SYS_FS_LIST
  Input: R1:R2 destination buffer. Copies the directory block (block 0) from disk into RAM via MEMCPY.

• 0x0A — SYS_FS_FIND
  Input: R1:R2 filename pointer.
  Output: R0 status (0 success, 1 not found), R1 block index, R2 file size.

• 0x0B — SYS_FS_READ
  Input: R1:R2 filename pointer, R3:R4 destination buffer.
  Output: R0 status (0 success, 1 not found). On success it loads the file into the buffer using the disk buffer window.

• 0x0C — SYS_FS_WRITE
  Currently unimplemented placeholder.

• 0x0D — SYS_FS_DELETE
  Currently unimplemented placeholder.

• 0x0E — SYS_EXEC
  Input: R1:R2 filename pointer. Loads the file into RAM at 0x1000 (user entry) and jumps to it.
  Output: R0 status (0 success, 1 not found).

• 0x0F — SYS_EXIT
  No inputs. Returns control to the kernel entrypoint at 0xE000 (used by user programs to quit).