Reading and printing strings from hard drives

.gitignore

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+boot.bin

Makefile

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+compile_and_run: compile run
+
+compile:
+	rm -rf boot.bin
+	nasm -f bin -o boot.bin main.asm
+
+run:
+	qemu-system-x86_64 -drive format=raw,file=boot.bin -drive format=raw,file=drive.bin -monitor stdio

main.asm

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+org 0x7C00                      
+bits 16                         
+
+jmp .start
+
+;;; Print string at bx
+.puts:
+.puts_loop:
+
+mov al, [bx]        ; load the character
+cmp al, 0           ; check for null byte
+je .end_puts_loop
+
+mov ah, 0x0e
+int 0x10            ; print charachter
+inc bx
+jmp .puts_loop
+.end_puts_loop:
+ret
+
+
+
+;;; Print bl as hex
+.puthex:
+
+; Print high bits
+mov dl, bl
+shr dl, 4
+
+cmp dl, 9
+jg .puthex_high_alpha
+
+; It's < 10, print as digit
+mov al, dl
+add al, 0x30
+mov ah, 0x0e
+int 0x10
+jmp .puthex_low
+
+.puthex_high_alpha:
+; It's >= 10, print as letter
+mov al, dl
+add al, 0x57 ; 0x57 = (0x61 - 10)
+mov ah, 0x0e
+int 0x10
+
+.puthex_low:
+; Print low bits
+mov dl, bl
+and dl, 0b1111
+
+cmp dl, 9
+jg .puthex_low_alpha
+
+; It's < 10, print as digit
+mov al, dl
+add al, 0x30
+mov ah, 0x0e
+int 0x10
+jmp .puthex_end
+
+.puthex_low_alpha:
+; It's >= 10, print as letter
+mov al, dl
+add al, 0x57 ; 0x57 = (0x61 - 10)
+mov ah, 0x0e
+int 0x10
+
+.puthex_end:
+ret
+
+
+
+.start:
+
+; reset disk system
+mov ah, 0x00
+mov dl, 0x81
+int 0x13
+
+; print result on error
+jnc .reset_disk_no_error
+mov bl, ah
+call .puthex
+.reset_disk_no_error:
+
+; Read sector 1 into memory
+mov ah, 0x02
+mov al, 0x01    ; number of sectors
+mov ch, 0x00    ; cylinder number, low 8 bits
+mov cl, 0x01    ; cylinder number, high 2 bits + sector number (6 bits)
+mov dh, 0x00    ; head number
+mov dl, 0x81    ; drive number
+mov bx, .data   ; data buffer
+int 0x13
+
+; print result on error
+jnc .read_disk_no_error
+mov bl, ah
+call .puthex
+.read_disk_no_error:
+
+; print string
+mov bx, .data
+call .puts
+
+.end:
+hlt
+
+.data:
+
+times 510 - ($ - $$) db 0
+dw 0xAA55