bryoboot1.asm

; ---------------------------[bryoboot1.asm]--------------------
; 1. constants
; 2. directives
;       + fat12 header
; 3. main
; 4. functions
; 5. messages
; 6. footer


; ---------------------------[CONSTANTS]------------------------
BL1_ADDR equ 0x7c00         ; origin address of loader1

BL2_ADDR equ 0x7fee         ; origin address of loader2
BL2_c equ 0                 ; cylinder for bl2
BL2_h equ 0                 ; head for bl2
BL2_s equ 2                 ; sector for bl2
BL2_sec_count equ 1         ; Number of sectors to read
;   Drive number is autoset by BIOS

CR equ 0x0d                 ; Carriage Return
LF equ 0x0a                 ; Line feed/ Newline

;----------------------------[DIRECTIVES]------------------------
org BL1_ADDR                ; origin
bits 16                     ; real mode 16 bit

;----------------------------[FAT12 HEADER]----------------------

jmp main                    ; First two bytes - jump to main
nop                         ; Null byte making the begining 3 bytes

bpb_oem: db 'bryoboot'       ; oem header 8 bytes
bpb_bps: dw 512              ; bytes per sector
bpb_sps: db 1                ; sectors per cluster
bpb_rs:  dw 1                ; reserved sector - 1 for bryoboot1
bpb_fc:  db 2                ; number of file allocation tables
bpb_dir_ent: dw 0xe0         ; number of dir entries
bpd_ts: dw 2880              ; Total sectors
bpb_md: dw 0xf0              ; Type of disk/Media descriptor
bpb_spt: dw 18               ; Sectors per track
bpb_hc: dw 2                 ; head count
bpb_hsc: dw 0                ; hidden sector count
bpb_lsc: dw 0                ; Large sector count

drive_num: dw 0              ; Drive number, floppy 0
db 0                         ; reserved
signature: db  0x29
volume_id:
    db 0x0,0x0,0x0,0x0       ; volume id - irrelevant to functionality
volume_label:
     db ' BRYO BOOT '        ; space padded 11 bytes
sysID: db 'FAT12   '         ; space padded 8 bytes

;----------------------------[MAIN]------------------------------
main:
    ; Segmentation registers setup
    xor ax,ax
    mov ds,ax
    mov es,ax
    mov ss,ax

    ; Printing Initial Boot message
    mov si,initial_boot_msg
    call print

    ; Attempt to load bl2
    call bl2_loader

    jmp $


;----------------------------[FUNCTIONS]--------------------------

print:
    .loop:
        lodsb
        or al,al
        jz .break
        mov ah,0x0e
        int 0x10
        jmp .loop

    .break:
        ret

bl2_loader:
    mov al,0x02               ; BIOS Read function
    mov al,BL2_sec_count      ;  How many Sectors to read

    ; CHS address
    mov ch,BL2_c
    mov dh,BL2_h
    mov cl,BL2_s

    mov bx,BL2_ADDR/16        ; Loading second stage address
    mov es,bx                 ; Pointing to extra segment from disk with es
    xor bx,bx
    int 0x13

    jc .drive_read_err
    jmp BL2_ADDR/16 : 0       ; Jumping to second loader

    .drive_read_err:
        mov si, drive_error_msg
        call print
        jmp $



;-----------------------------[MESSAGES]--------------------------
initial_boot_msg:
     db '[BL1] BRYOBOOT loaded sucessfully',CR,LF,0
drive_error_msg:
     db '[BL1] DISK READING ERROR - Couldnt Load BL2',CR,LF,0

;-----------------------------[FOOTER]----------------------------
times 510-($-$$) db 0        ; Padding to 512 bytes
dw 0xaa55                    ; Magic Number for BIOS identification