kernel: First steps of calculating drive tab data from BPB
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@ -2,9 +2,28 @@
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org 0x500
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%include "inc/bpb.asm"
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%include "inc/mbr.asm"
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%include "inc/dpt.asm"
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; drive table entry
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struc drive
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.biosnum: resb 1
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.flag: resb 1
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.spc: resw 1
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.cylinders: resw 1
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.dpt: resb dpt_size
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.type: resb 1 ; 12 or 16, depending on FAT type
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.fat_offset: resd 1 ; offset of fat table
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.fat_num: resw 1 ; number of fat tables
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.fat_size: resw 1 ; sectors per fat table
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.dir_offset: resd 1 ; offset of root directory
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.dir_size: resw 1 ; size of root directory in sectors
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.clus_offset: resd 1 ; offset of clusters
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.clus_num: resw 1 ; number of clusters
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.clus_size: resw 1 ; sectors per cluster
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endstruc
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; kernel stack size in words
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%define stacksize 512
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@ -240,6 +259,7 @@ fputc: push ax
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iret
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%include "kernel/drive.asm"
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%include "kernel/fat.asm"
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zero: dw 0
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@ -2,15 +2,6 @@ section .bss
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drives: equ 16
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; drive table entry
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struc drive
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.biosnum: resb 1
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.flag: resb 1
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.spc: resw 1
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.cylinders: resw 1
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.dpt: resb dpt_size
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endstruc
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; ptr into currently selected drive
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drive_ptr: resw 1
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102
kernel/fat.asm
102
kernel/fat.asm
@ -1,111 +1,59 @@
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section .bss
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section .text
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; the sector numbers have the partition offset already included
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fat_offset:
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resd 1
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fat_size:
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resd 1
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fat_num: ; a byte would be enough, but i want to work with cx
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resw 1
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rootdir_offset:
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resd 1
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rootdir_size:
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resw 1
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clusters_offset:
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resd 1
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clusters_num:
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resd 1
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section .text
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load_ts: ; load total number of sectors into DX:AX
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xor dx, dx
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mov ax, [disk_buffer+0x013]
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test ax, ax
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jnz .ret
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mov ax, [disk_buffer+0x020]
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mov dx, [disk_buffer+0x022]
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.ret: ret
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; invoked after disk select
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load_bpb:
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load_bpb: mov bx, [drive_ptr]
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; read word for "sectors per fat"
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mov ax, [disk_buffer+0x016]
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mov ax, [diskbuf+0x016]
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xor dx, dx
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; if zero, we need to check dword in BPB 7.1
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test ax, ax
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jnz .short_sf
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; fail if BPB 7.1 signature is not present
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mov cl, [disk_buffer+0x042]
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mov cl, [diskbuf+0x042]
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or cl, 1
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cmp cl, 0x29
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jne .err
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; load dword
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mov ax, [disk_buffer+0x024]
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mov dx, [disk_buffer+0x026]
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mov ax, [diskbuf+0x024]
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mov dx, [diskbuf+0x026]
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.short_sf:
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; store local value
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mov [fat_size], ax
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mov [fat_size+2], dx
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mov [bx+drive.fat_size], ax
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mov [bx+drive.fat_size+2], dx
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; copy number of fat's
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xor ah, ah
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mov al, [disk_buffer+0x010]
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mov [fat_num], ax
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mov al, [diskbuf+0x010]
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mov [bx+drive.fat_num], ax
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; copy number of root directory entries
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mov ax, [disk_buffer+0x011]
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mov ax, [diskbuf+0x011]
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mov cl, 4
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shr ax, cl
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mov [rootdir_size], ax
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mov [bx+drive.dir_size], ax
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; calculate offsets for everything
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mov ax, [part_offset]
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mov dx, [part_offset+2]
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xor ax, ax
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xor dx, dx
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; add reserved sectors
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add ax, [disk_buffer+0x0B+bpb_rsc]
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add ax, [diskbuf+0x0B+bpb_rsc]
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adc ax, 0
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; save begin of FAT tables
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mov [fat_offset], ax
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mov [fat_offset+2], dx
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mov [bx+drive.fat_offset], ax
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mov [bx+drive.fat_offset+2], dx
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; add FAT size * FAT number
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mov cx, [fat_num]
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.floop: add ax, [fat_size]
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adc dx, [fat_size+2]
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mov cx, [bx+drive.fat_num]
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.floop: add ax, [bx+drive.fat_size]
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adc dx, [bx+drive.fat_size+2]
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loop .floop
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; save begin of root directories
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mov [rootdir_offset], ax
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mov [rootdir_offset+2], dx
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mov [bx+drive.dir_offset], ax
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mov [bx+drive.dir_offset+2], dx
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; add root directory size (might be zero)
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add ax, [rootdir_size]
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add ax, [bx+drive.dir_size]
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adc dx, 0
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; save start of data area
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mov [clusters_offset], ax
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mov [clusters_offset+2], dx
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call load_ts
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add ax, [part_offset]
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adc dx, [part_offset+2]
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sub ax, [clusters_offset]
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sbb dx, [clusters_offset+2]
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; get "sectors per cluster"
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mov cl, [disk_buffer+0x00D]
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; this turns dx:ax from data sectors to number of clus
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.sloop: shr cl, 1
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test cl, cl
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jz .send
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clc
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rcr dx, 1
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rcr ax, 1
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jmp .sloop
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.send: mov [clusters_num], ax
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mov [clusters_num+2], dx
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int 3
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mov [bx+drive.clus_offset], ax
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mov [bx+drive.clus_offset+2], dx
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ret
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.err: stc
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