SLRboot/slrboot.nasm

ORG 0x7C00
BITS 16
;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
; SLRboot
; Bootloader for SingOS
; version 0.2.1.0-exp
;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
cli
jmp long 0x0000:start

start:
mov ax, 0x1000
mov ss, ax
mov sp, 0xb000
mov ax, 0
mov ds, ax
mov es, ax
mov [disk_identifier], dl
sti
mov si, 0x7c00
mov di, 0x600
mov cx, 0x100
rep movsw
mov bx, relocated
sub bx, 0x7600 ; Calculate the offset.
push bx
ret ; Return far
;jmp long 0:bx

relocated:

mov si, message ; Put address of the null-terminated string to output into 'si'
call print      ; Call our string-printing routine

; Check for more than one active partition that we can start from. 
xor bx, bx
xor cx, cx
mov ax, [partition_1.active_partition]
cmp al, 0x80
jne check_p_2
mov bx, 0x01 ; Partion 1 active
inc cx

check_p_2:
mov ax, [partition_2.active_partition]
cmp al, 0x80
jne check_p_3
inc cx
mov bx, 0x02 ; Partion 2 active

check_p_3:
mov ax, [partition_3.active_partition]
cmp al, 0x80
jne check_p_4
inc cx
mov bx, 0x03 ; Partion 3 active

check_p_4:
mov al, [partition_4.active_partition]
cmp ax, 0x80
jne eval_active_partion_number
inc cx
mov bx, 0x04 ; Partion 4 active
cli
hlt
eval_active_partion_number: 
cmp cx, 1

je boot_partition
; here goes wait call, for the user to enter debug mode. 
; Wating for 2 seconds: 
mov ah, 0x86   ; code for waiting interupt call
mov cx, 0x001e ; high count of microseconds
mov dx, 0x8480 ; low count
int 0x15
.busy_wait_for_key:
xor ax, ax 
mov ah, 0x01 ; BIOS call to wait for key
int 0x16

boot_partition:
; IMPORTANT bx, has to hold the value 1-4 for the partiotion that wanted to be booted.  

mov ax, partition_1
.loop:
    cmp bx, 1 
    je break
    add ax, 0x10
    dec bx
    jmp .loop
break:

add ax, 8 ; This is the offset to fetch the LBA start adress of the partition record
mov bx, ax
xor ax, ax
mov ax, [bx] ; first part of LBA
mov WORD [DAPACK.lba_addr_dw_low], ax
call dumpax
add bx, 2 ; next part
mov ax, [bx]
mov WORD [DAPACK.lba_addr_dw_low + 1], ax
call dumpax
mov WORD [DAPACK.blkcnt],          0x01  
mov WORD [DAPACK.db_addr_segment], 0x0000
mov WORD [DAPACK.db_addr_offset],  0x7c00
mov si, DAPACK  ; address of "disk address packet"
xor ax, ax                          
mov ah, 0x42                            ; READ
mov dl, [disk_identifier] 
int 0x13

jc endcarrycheck ; An error ocurred when reading from disk. 

; If no error then jump to volume boot record
notcarry: 
jmp long 0x0:0x7c00

cli 
hlt

; If we did not suceed to read from disk
endcarrycheck:  
mov si, error_str
call print
cli             ; Clear the Interrupt Flag (disable external interrupts)
hlt             ; Halt the CPU (until the next external interrupt)

debug_mode: 
mov si, welcome_debug
call print 
cli             ; Clear the Interrupt Flag (disable external interrupts)
hlt 

;Routine for printing a 'ax' as hex
dumpax:
pusha           ; save registers
mov bx, ax
mov ah, 0xE     ; Teletype output

mov cx, 4       ; 4 nipples in a 16 bit word
.loop:
rol bx, 4       ; rotate to next nipple
mov al, bl      ; we copy to al because we need to mask only the low 4 bits
and al, 1111b   ; Do the masking
add al, '0'     ; convert to ASCII
cmp al, '9'     ; If we are greater than 9 ascii, we add 7 to make digit 10 be represented as 'A'
jbe .skip       ;  -|-
add al, 7       ;  -|-
.skip:          ;  -|-
int 0x10        ; BIOS call 'output'
loop .loop

popa            ; restore registers
ret

printCRLF:
mov ah, 0xE
mov al, 13
int 0x10
mov al, 10
int 0x10
ret

; Routine for outputting string in 'si' register to screen
print:
mov ah, 0xE     ; Specify 'int 0x10' 'teletype output' function
                ; [AL = Character, BH = Page Number, BL = Colour (in graphics mode)]
.printchar:
lodsb           ; Load byte at address SI into AL, and increment SI
cmp al, 0
je .done        ; If the character is zero (NUL), stop writing the string
int 0x10         ; Otherwise, print the character via 'int 0x10'
jmp .printchar  ; Repeat for the next character
.done:
ret

data:
message db 'SLRboot for SingOS! v0.2.1.0-exp',13,10,0
enter_debug_mode db 'Press d to enter bootloader debug mode',13,10,0
welcome_debug db 'Debug mode v.0.0.1:',13,10,0
error_str db 'Error', 0
boot_this_partition: dw 0
disk_identifier db 0


DAPACK:
  .dap_Size:         db  0x10   ; This is always 16 bytes (0x10)
  .rev_byte:         db  0x0    ; reserved byte, should always be zero
  .blkcnt:           dw  0x0    ; int 13 resets this to # of blocks actually read/written
  .db_addr_offset:   dw  0x0    ; memory buffer destination address (0:7c00)
  .db_addr_segment:  dw  0x0    ; in memory page zero
  .lba_addr_dw_low:  dd  0x0    ; put the lba to read in this spot
  .lba_addr_dw_high: dd  0x0    ; more storage bytes only for big lba's ( > 4 bytes ) 


; Pad to 510 bytes (boot sector size minus 2) with 0s, and finish with the two-byte standard boot signature
times 446-($-$$) db 0 ; First partion entry
partition_1:
.active_partition db 0x80 
.start_CHS db 0x00, 0x00, 0x11;0x20, 0x21, 0x00 
.disk_type db 0x83 
.end_CHS db 0x8C, 0x3D, 0x0F 
.start_LBA db 0x10, 0x00, 0x00, 0x00 
.size db 0x00, 0xC8, 0x03, 0x00  

partition_2:
.active_partition db 0x00 
.start_CHS db 0x8C, 0x3E, 0x0F 
.disk_type db 0x83 
.end_CHS db 0xFF, 0xFF, 0xFF 
.start_LBA db 0x00, 0xD0, 0x03, 0x00 
.size db 0x00, 0x28, 0x1C, 0x03 

partition_3:
.active_partition db 0x00 
.start_CHS db 0x00, 0x00, 0x00 
.disk_type db 0x00 
.end_CHS db 0x00, 0x00, 0x00 
.start_LBA db 0x00, 0x00, 0x00, 0x00 
.size db 0x00, 0x00, 0x00, 0x00 

partition_4:
.active_partition db 0x00 
.start_CHS db 0x00, 0x00, 0x00 
.disk_type db 0x00 
.end_CHS db 0x00, 0x00, 0x00 
.start_LBA db 0x00, 0x00, 0x00, 0x00 
.size db 0x00, 0x00, 0x00, 0x00

times 510-($-$$) db 0
dw 0xAA55           ; => 0x55 0xAA (little endian byte order)
; bootloder debug_mode goes here

times 8192-($-$$) db 0