fox32rom/boot.asm
Ry 4fc5324f2a fox32+fox32rom: Use DMA for the disk controller
Instead of accessing one byte at a time, use a DMA-like system to
read/write a whole sector at a time directly to/from memory.
2022-03-12 13:12:10 -08:00

57 lines
1.9 KiB
NASM

; disk booting routines
; these are only used during booting, they are not exposed via the jump table
; read disk 0 and attempt to figure out what type of disk it is, and load the correct binary into memory
; inputs:
; r0: disk size (bytes)
; outputs:
; none (doesn't return)
start_boot_process:
; r0 contains the size of the disk in bytes
; divide the size by 512 and add 1 to get the size in sectors
div r0, 512
inc r0
mov r31, r0
mov r0, 0 ; sector counter
mov r2, 0x00000800 ; destination pointer
mov r3, 0x80003000 ; command to read a sector from disk 0 into memory
mov r4, 0x80002000 ; command to set the location of the buffer
; first, check to see if this is a FAT-formatted disk
out r4, r2 ; set the memory buffer location
out r3, 0 ; read sector 0 into the buffer
cmp.8 [r2], 0xEB ; check for an x86 jmp instruction, indicating a possible FAT volume
ifz jmp start_boot_process_fat
cmp.8 [r2], 0xE9 ; check for an x86 jmp instruction, indicating a possible FAT volume
ifz jmp start_boot_process_fat
start_boot_process_raw_binary_sector_loop:
out r4, r2 ; set the memory buffer location
out r3, r0 ; read the current sector into memory
inc r0 ; increment sector counter
add r2, 512 ; increment the destination pointer
loop start_boot_process_raw_binary_sector_loop
jmp start_boot_process_done
start_boot_process_fat:
;
start_boot_process_done:
; done loading !!!
; now clean up and jump to the loaded binary
call boot_cleanup
jmp 0x00000800
; clean up the system's state before jumping to the loaded binary
; inputs:
; none
; outputs:
; none
boot_cleanup:
; clear the background
mov r0, BACKGROUND_COLOR
call fill_background
; disable the menu bar
mov r0, 0x8000031E
out r0, 0
ret