+; org: all instructions from now on start in 7c00h
+; 7c00h is where the bios loads the 1st sector
+; assembler formatting:
+; column 0: labels
+; tab: commands
+ org 7c00h
+; at boot: real mode (like it's 8086)
+; we have to tell assembler that code is 16bit mode
+ bits 16
+
+start:
+ mov sp, 7c00h
+
+; service 0: set videomode ah
+; param: which video mode (13h: 320x200, 256 colors) al
+; ax: ah (high 8 bits), al (low)
+ mov ax, 13h
+
+; calling video bios
+; software interrupt 10h
+ int 10h
+ call clearscreen
+
+; infinite loop
+end:
+ jmp end
+
+clearscreen:
+; video ram is mapped in a0000
+; first 64000 bytes appear immediately on screen
+; mem addresses in real mode have 2 parts: segment and offset
+; bits overlap: segment is shifted by 4 and is added to the
+; overlapping offset (20 bits number = x86's addressable space = 1MB)
+; segment register for the segment: es, ds, cs, ss (and: fs, gs)
+; default register = ds (data segment), es = extra segment
+; cs = code segment cs:ip (it points where to read instructions and is
+; automatically set to 7c0 (7c00h)
+; offset can be paired with any register
+; pair the registers
+ mov ax, 0a000h
+ mov ds, ax
+ mov di, 0
+; counter cx
+ mov cx, 64000
+ mov ax, 3
+.loop_begin:
+; dereferrence[] address
+ mov [di], al
+ inc di
+ dec cx
+; when cx is 0, 0 flag is set
+ jnz .loop_begin
+ ret
+
+; assembler trick: write as many 0 needed to fill 510 bytes
+; $ <- means here
+ times 510-($-$$) db 0
+ dw 0aa55h
projects / bootboot / commitdiff