replaced the first 64 colors of the palette with grayscale

[bootboot] / bb.asm

diff --git a/bb.asm b/bb.asm
index f5c84af..3f4bcd1 100644 (file)
--- a/bb.asm
+++ b/bb.asm
@@ -11,19 +11,60 @@
 start:
        mov sp, 7c00h
 
+; initialize segment registers
+       mov ax, 0
+       mov ds, ax
+       mov es, ax
+       mov ss, ax
+
+; save dl value
+; bios sets in dl the num of the drive that
+; loaded you so if you overwrite dl you can reuse it
+       mov [saved_drive_num], dl
+
 ; service 0: set videomode ah
 ; param: which video mode (13h: 320x200, 256 colors) al
 ; ax: ah (high 8 bits), al (low)
+; call 0 = set_videomode it expects the video mode in al
+; ah : 0, al: 13, ax:ahal
        mov ax, 13h
 
 ; calling video bios
 ; software interrupt 10h
+; what's the value of ah? al=video mode when ah=0
+; which set which video mode
        int 10h
+       mov ax, 3
        call clearscreen
 
-; infinite loop
-end:
-       jmp end
+; waiting for keypress
+; x86 has 2 special instructions to read/write from I/O
+; devices: in and out (because some processors have different
+; address spaces for devices and for memory, arm not)
+.key_wait:
+       in al, 64h      ; 60h = keyb data port, 64h = keyb status port
+       and al, 1       ; 1 = OUTBUF_FULL = the keyb controller out buf is full
+       jz .key_wait
+
+       in al, 60h      ; reads the keyb that was pressed to reset the flag
+
+; load 2nd sector from boot device and jump to it
+; bios helpers, 13h = disk io
+       mov ax, 0
+       mov ds, ax
+       mov ah, 02h                     ; call 2: read sectors into memory
+       mov al, 1                               ; number of sectors to read
+       mov ch, 0                               ; low 8 bits of cylinder number
+       mov cl, 2                               ; sector number that starts from 1
+       mov dh, 0                               ; head number
+       mov dl, [saved_drive_num]       ; 8bits
+       mov bx, sector_2
+       int 13h
+; error check: if carry flag isn't set jump to loaded code
+       jnc     sector_2
+.inf_loop:
+       jmp .inf_loop
+
 
 clearscreen:
 ; video ram is mapped in a0000
@@ -37,12 +78,14 @@ clearscreen:
 ; automatically set to 7c0 (7c00h)
 ; offset can be paired with any register
 ; pair the registers
+       push ax
        mov ax, 0a000h
        mov ds, ax
        mov di, 0
+       pop ax
 ; counter cx
        mov cx, 64000
-       mov ax, 3
+
 .loop_begin:
 ; dereferrence[] address
        mov [di], al
@@ -52,7 +95,53 @@ clearscreen:
        jnz .loop_begin
        ret
 
+saved_drive_num:
+       db 0 ; define byte 0
+
 ; assembler trick: write as many 0 needed to fill 510 bytes
 ; $ <- means here
        times 510-($-$$) db 0
        dw 0aa55h
+
+sector_2:
+; disp palette
+       mov ax, 0a000h  ; video segment points to video memory
+       mov ds, ax
+       mov bx, 0               ; video offset
+       mov cx, 0               ; y
+.y_loop:
+       mov dx, 0               ; x
+.x_loop:
+       mov ax, dx
+       test ax, 0ff00h ; lower 8 bits 0, highest 1, Z flag is set while < 256, test = and but doesnt change ax 
+       jz      .skip_clear_ax
+       mov al, 0
+.skip_clear_ax:
+       mov [bx], al    ; pixel written on screen
+       inc bx                  ; increment
+       inc dx
+       cmp dx, 320
+       jnz .x_loop
+       inc cx
+       cmp cx, 200
+       jnz .y_loop
+
+; setup grayscale palette (64 first colors because ramdac uses 6 bits / color so 2^6)
+; ram dac (digital to analog converter) tis vga
+; in a ^ register which index we want to write
+; 3 writes in another ramdac register (data)
+; ramdac feature: when you need to set a palette
+       mov al, 0               ; first index
+       mov dx, 3c8h    ; ramdac index registe
+       out dx, al              ; because io port (address) > 255
+       inc dx                  ; ramdac data register: 3c9h
+.pal_loop:
+       out dx, al              ; r
+       out dx, al              ; g
+       out dx, al              ; b
+       inc al
+       test al, 3fh    ; test with 3fh to keep the lowest 6 bits of al
+       jnz .pal_loop
+
+.inf_loop:
+       jmp .inf_loop