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fox32+fox32rom: Use DMA for the disk controller

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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.
This commit is contained in:
Ry 2022-03-12 13:12:10 -08:00
parent e332558e21
commit 4fc5324f2a
1 changed files with 19 additions and 20 deletions
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39
boot.asm
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@ -2,16 +2,11 @@
; 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
; currently this only supports booting raw binaries
; inputs:
; r0: disk size (bytes)
; outputs:
; none (doesn't return)
start_boot_process:
; in the future, this will check the header for various different types of disk types
; but for now, just assume the user inserted a raw binary
; load it to 0x00000800 (immediately after the interrupt vectors) and jump
; 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
@ -20,22 +15,26 @@ start_boot_process:
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 the sector buffer
mov r4, 0x80002000 ; command to read a byte from the sector buffer
start_boot_process_sector_loop:
out r3, r0 ; read the current sector into the sector buffer
mov r1, 0 ; byte counter
start_boot_process_byte_loop:
mov r5, r4
or r5, r1 ; or the byte read command with the current byte counter
in r5, r5 ; read the current byte into r5
mov.8 [r2], r5 ; write the byte
inc r2 ; increment the destination pointer
inc r1 ; increment the byte counter
cmp r1, 512
ifnz jmp start_boot_process_byte_loop
loop start_boot_process_sector_loop
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