From 27427fd7f284b5359def788acbccff31989cdecf Mon Sep 17 00:00:00 2001 From: Eleni Maria Stea Date: Fri, 4 Dec 2020 18:50:17 +0200 Subject: [PATCH] step 2: load 2nd sector and jump into it --- .gdbinit | 4 ++++ Makefile | 8 ++++++++ bb.asm | 43 +++++++++++++++++++++++++++++++++++++++---- 3 files changed, 51 insertions(+), 4 deletions(-) create mode 100644 .gdbinit diff --git a/.gdbinit b/.gdbinit new file mode 100644 index 0000000..a3fac16 --- /dev/null +++ b/.gdbinit @@ -0,0 +1,4 @@ +target remote localhost:1234 +set architecture i8086 +set disassembly-flavor intel +disp/i $pc diff --git a/Makefile b/Makefile index e52a97e..ea23f67 100644 --- a/Makefile +++ b/Makefile @@ -8,6 +8,10 @@ floppy.img: $(bin) dd if=/dev/zero of=$@ bs=1024 count=1440 dd if=$< of=$@ bs=512 conv=notrunc +.PHONY: disasm +disasm: $(bin) + ndisasm -b16 -o 0x7c00 $< > dis + .PHONY: clean clean: rm -f $(bin) @@ -16,3 +20,7 @@ clean: .PHONY: run run: floppy.img qemu-system-i386 -fda $< + +.PHONY: debug +debug: floppy.img + qemu-system-i386 -S -s -fda $< diff --git a/bb.asm b/bb.asm index f5c84af..b3ef814 100644 --- a/bb.asm +++ b/bb.asm @@ -11,6 +11,15 @@ start: mov sp, 7c00h +; initialize segment registers + mov ax, 0 + mov ds, ax + mov es, ax + mov ss, ax + +; save dx value + 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) @@ -19,11 +28,26 @@ start: ; calling video bios ; software interrupt 10h int 10h + mov ax, 3 call clearscreen -; infinite loop -end: - jmp end +; 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 +61,14 @@ 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 +78,16 @@ clearscreen: jnz .loop_begin ret +saved_drive_num: + db 0 ; declare byte 0 + ; assembler trick: write as many 0 needed to fill 510 bytes ; $ <- means here times 510-($-$$) db 0 dw 0aa55h + +sector_2: + mov ax, 5 + call clearscreen +.inf_loop: + jmp .inf_loop -- 1.7.10.4