From: Eleni Maria Stea <elene.mst@gmail.com>
Date: Fri, 4 Dec 2020 15:53:38 +0000 (+0200)
Subject: step 1: boot from floppy and clear screen
X-Git-Url: https://eleni.mutantstargoat.com/git/?p=bootboot;a=commitdiff_plain;h=ed7544d82195f704b31bf78cb9ae1ca967b17a1f

step 1: boot from floppy and clear screen
---

ed7544d82195f704b31bf78cb9ae1ca967b17a1f
diff --git a/Makefile b/Makefile
new file mode 100644
index 0000000..e52a97e
--- /dev/null
+++ b/Makefile
@@ -0,0 +1,18 @@
+bin = bootboot
+# build flat binary (no headers)
+ASFLAGS = -f bin
+$(bin): bb.asm
+	nasm $(ASFLAGS) -o $@ $<
+
+floppy.img: $(bin)
+	dd if=/dev/zero of=$@ bs=1024 count=1440
+	dd if=$< of=$@ bs=512 conv=notrunc
+
+.PHONY: clean
+clean:
+	rm -f $(bin)
+
+# floppy disk A (fda)
+.PHONY: run
+run: floppy.img
+	qemu-system-i386 -fda $<
diff --git a/bb.asm b/bb.asm
new file mode 100644
index 0000000..f5c84af
--- /dev/null
+++ b/bb.asm
@@ -0,0 +1,58 @@
+; 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