From 3ca4ce76f0e1700a136a9eb93fea10a3b4b35027 Mon Sep 17 00:00:00 2001 From: Tibo Date: Mon, 4 Oct 2021 17:59:48 +0200 Subject: [PATCH] add documentation --- messages.asm | 281 +++++++++++++++++++++++++++------------------------ 1 file changed, 150 insertions(+), 131 deletions(-) diff --git a/messages.asm b/messages.asm index 17f188c..6446158 100644 --- a/messages.asm +++ b/messages.asm @@ -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