add documentation

This commit is contained in:
Tibo 2021-10-04 17:59:48 +02:00
parent a8a7a3d3d1
commit 3ca4ce76f0

View file

@ -1,9 +1,11 @@
org 0x7C00
bits 16
org 0x7C00 ; The boot code on a drive always gets loaded at address 0x7C00
bits 16 ; .bin defaults to 16 bits
WIDTH equ 0x50
HEIGHT equ 0x18
VIDEO_MODE equ 0x3
;; FIXME: possible problem with HEIGHT being defined
;; as one less than what VIDEO_MODE implies?
WIDTH equ 0x50 ; 60
HEIGHT equ 0x18 ; 24 (?)
VIDEO_MODE equ 0x3 ; 80x25 screen, text mode
;; see http://www.ctyme.com/intr/int-10.htm for interrupts
@ -13,173 +15,190 @@ mov ax, VIDEO_MODE
int 10h
;; Clear screen
mov ah, 0x06
mov al, 0x00
mov bh, 0x0F
mov cx, 0x0000
mov dh, HEIGHT
mov dl, WIDTH
mov ah, 0x06 ; scroll screen up
mov al, 0x00 ; lines to scroll (0 = clear)
mov bh, 0x0F ; bg/fg colour
mov cx, 0x0000 ; upper row number, left col number (both 0)
mov dh, HEIGHT ; lower row number (24)
mov dl, WIDTH ; right col number (80)
int 0x10
;; Set cursor to bottom of screen
mov dh, HEIGHT
mov dl, 0x00
mov bh, 0x00
mov ah, 0x02
mov ah, 0x02 ; set cursor position
mov bh, 0x00 ; page number
mov dh, HEIGHT ; row (bottommost row)
mov dl, 0x00 ; col (leftmost col)
int 0x10
mov sp, 0x2000
mov sp, 0x2000 ; initialise stack pointer
.loop:
;; Read character
mov ah, 0x00
int 0x16
mov ah, 0x00 ; Read character
int 0x16
cmp al, 0x0d ; newline
je .newline
cmp al, 0x08 ; delete character
je .change_color_mode
cmp al, 0x1b ; escape character
je .beep
jmp .nonewline
cmp al, 0x0d ; newline
je .newline
cmp al, 0x08 ; delete character
je .change_color_mode
cmp al, 0x1b ; escape character
je .beep
jmp .nonewline
.change_color_mode:
call .readbyte
mov [color_mode], al
jmp .loop
call .readbyte
mov [color_mode], al
jmp .loop
.newline:
;; Scroll up window
mov ah, 0x06
mov al, 0x01
mov bh, 0x0F
mov cx, 0x0000
mov dh, HEIGHT
mov dl, WIDTH
int 0x10
;; Scroll up window
mov ah, 0x06
mov al, 0x01
mov bh, 0x0F
mov cx, 0x0000
mov dh, HEIGHT
mov dl, WIDTH
int 0x10
;; Get current cursor position
mov bh, 0x00
mov ah, 0x03
int 0x10
;; Get current cursor position
mov bh, 0x00
mov ah, 0x03
int 0x10
;; Move cursor to beginning of screen
mov dl, 0x00
mov ah, 0x02
int 0x10
;; Move cursor to beginning of screen
mov dl, 0x00
mov ah, 0x02
int 0x10
jmp .loop
jmp .loop
.beep:
call .readduration
mov ah, 0
push ax
mov al, 182 ; Prepare the speaker for the
out 43h, al ; note.
call .readbyte
mov ah, 0
sal ax, 5 ; multiply by 32
add ax, 1140
out 42h, al ; Output low byte.
mov al, ah ; Output high byte.
out 42h, al
in al, 61h ; Turn on note (get value from
; port 61h).
or al, 00000011b ; Set bits 1 and 0.
out 61h, al ; Send new value.
;; mov bx, 50000 ; Pause for duration of note.
pop bx
push ax
mov ax, 3125
mul bx
mov bx, ax
pop ax
call .readduration
mov ah, 0
push ax
mov al, 182 ; Prepare the speaker for the
out 43h, al ; note.
call .readbyte
mov ah, 0
sal ax, 5 ; multiply by 32
add ax, 1140
out 42h, al ; Output low byte.
mov al, ah ; Output high byte.
out 42h, al
in al, 61h ; Turn on note (get value from
; port 61h).
or al, 00000011b ; Set bits 1 and 0.
out 61h, al ; Send new value.
;; mov bx, 50000 ; Pause for duration of note.
pop bx
push ax
mov ax, 3125
mul bx
mov bx, ax
pop ax
.pause1:
mov cx, 65535
mov cx, 65535
.pause2:
dec cx
jne .pause2
dec bx
jne .pause1
in al, 61h ; Turn off note (get value from
; port 61h).
and al, 11111100b ; Reset bits 1 and 0.
out 61h, al ; Send new value.
jmp .loop
dec cx
jne .pause2
dec bx
jne .pause1
in al, 61h ; Turn off note (get value from
; port 61h).
and al, 11111100b ; Reset bits 1 and 0.
out 61h, al ; Send new value.
jmp .loop
.nonewline:
;; Write character
;; Write character
mov ah, 0x09
mov bh, 0x00
mov bl, [color_mode]
mov cx, 0x01
mov bh, 0x00
mov bl, [color_mode]
mov cx, 0x01
int 0x10
;; Get current cursor position
mov ah, 0x03
int 0x10
;; Get current cursor position
mov ah, 0x03
int 0x10
cmp dl, WIDTH
jge .eol
jmp .noeol
cmp dl, WIDTH
jge .eol
jmp .noeol
.eol:
;; Scroll up window
mov ah, 0x06
mov al, 0x01
mov bh, 0x0F
mov cx, 0x0000
mov dh, HEIGHT
mov dl, WIDTH
int 0x10
;; Scroll up window
mov ah, 0x06
mov al, 0x01
mov bh, 0x0F
mov cx, 0x0000
mov dh, HEIGHT
mov dl, WIDTH
int 0x10
;; Move cursor to beginning of screen
mov bh, 0
mov dh, HEIGHT
mov dl, 0x00
mov ah, 0x02
int 0x10
jmp .loop
;; Move cursor to beginning of screen
mov bh, 0
mov dh, HEIGHT
mov dl, 0x00
mov ah, 0x02
int 0x10
jmp .loop
.noeol:
;; Move cursor forward
inc dl
mov ah, 0x02
int 0x10
jmp .loop
;; Move cursor forward
inc dl
mov ah, 0x02
int 0x10
jmp .loop
;; Reads two characters from the keyboard buffer and converts them into a single byte
;;
;; The conversion happens as follows:
;; ASCII 'a' is subtracted from each character (so 'a' = 0, 'b' = 1, ...)
;; the first character is shifted left by 4
;; the shifted and non-shifted values are then or'ed together to form a byte
;;
;; CLOBBERS
;; - ax
;; - bl
.readbyte:
;; Read character
mov ah, 0x00
int 0x16
;; Read character is in al
sub al, 'a'
mov bl, al
shl bl, 0x4
mov ah, 0x00 ; Read character
int 0x16
;; Read character
mov ah, 0x00
int 0x16
;; Read character is in al
sub al, 'a'
or al, bl
ret
;; Read character is in al
sub al, 'a' ; Normalise
mov bl, al
shl bl, 0x4 ; First character is upper 4 bits
mov ah, 0x00 ; Read character
int 0x16
;; Read character is in al
sub al, 'a' ; Normalise
or al, bl ; Combine both characters
ret
.readduration:
;; Read duration (0-16 supported)
mov ah, 0x00
int 0x16
;; Read character is in al
sub al, 'a'
ret
;; Read duration (0-16 supported)
mov ah, 0x00
int 0x16
;; Read character is in al
sub al, 'a'
ret
halt: hlt
color_mode: db 0x02
;; Magic number must be in the last 2 bytes of the sector so
;; fill the binary with zeroes until it is 510 bytes in size
;; (magic number will use up the remaining 2)
times 510 - ($ - $$) db 0
dw 0xAA55
dw 0xAA55 ;; Magic number to mark sector as bootable