OSDev.org

org 0x7c00                  ; BIOS load address

start:
    ; Initialize the processor (not explicitly done in the code, usually BIOS job)
    ; Initialize the serial port
    mov ah, 0x00            ; Set COM port initialization function
    mov al, 0x03            ; Baud rate divisor for 9600 baud
    mov dx, 0               ; COM1 port number (0 for COM1)
    int 0x14                ; Call BIOS COM interrupt

    ; Clear the screen
    mov ax, 0x0003          ; Set video mode to 80x25 text mode
    int 0x10

main_loop:
    call read_byte          ; Read a byte from the serial port
    cmp al, 0x01            ; Check if byte = 01 [fetch]
    je fetch_byte
    cmp al, 0x02            ; Check if byte = 02 [store]
    je store_byte
    cmp al, 0x03            ; Check if byte = 03 [call]
    je call_subroutine
    jmp main_loop           ; Loop back if none of the above

fetch_byte:
    call read_address       ; Get address from the serial port
    mov al, [bx]            ; Fetch the byte from that address
    call send_byte          ; Send the byte to the serial port
    jmp main_loop

store_byte:
    call read_address       ; Get address from the serial port
    call read_byte          ; Get a byte from the serial port
    mov [bx], al            ; Store the byte at that address
    jmp main_loop

call_subroutine:
    call read_address       ; Get address from the serial port
    call far [bx]           ; Call the subroutine at that address
    jmp main_loop           ; Return to the main loop after subroutine

read_address:               ; Function to read an address from the serial port
    call read_byte          ; Read low byte
    mov bl, al
    call read_byte          ; Read high byte
    mov bh, al
    ret

read_byte:                  ; Function to read a byte from the serial port
    mov dx, 0x3f8           ; COM1 base address
wait_for_data:
    in al, dx               ; Read line status register
    test al, 1              ; Test if data available (bit 0)
    jz wait_for_data        ; If not, loop until data is available
    mov dx, 0x3f8           ; Reset dx to point to the data register
    in al, dx               ; Read received byte from data register
    ret

send_byte:                  ; Function to send a byte to the serial port
    push ax                 ; Save AX register
    mov ah, al              ; Move byte to AH for sending
    mov dx, 0               ; COM1 port number (0 for COM1)
    int 0x14                ; Call BIOS COM interrupt
    pop ax                  ; Restore AX register
    ret

; The print_byte and nibble_to_ascii functions are not necessary for the goals
; and have been removed to simplify the code.

times 510-($-$$) db 0       ; Pad remaining bytes with zeros
dw 0xaa55                   ; Boot signature

; boot.fasm

use16
org 0x7C00

start:
    cli             ; Disable interrupts
    mov ax, 0x07C0  ; Set up the stack
    mov ss, ax
    mov sp, 0x1000
    sti             ; Enable interrupts

    call init_uart  ; Initialize UART for serial communication

    ; Set video mode to 80x25 text mode
    mov ah, 0x00
    mov al, 0x03
    int 0x10

    ; Main loop to echo characters
main_loop:
    call read_byte_from_com1  ; Read a byte from COM1
    call print_char           ; Print the received character
    jmp main_loop             ; Repeat indefinitely

; Include the serial communication routines
include 'com.fasm'

; Print a character to the screen
print_char:
    mov ah, 0x0E   ; BIOS teletype output
    mov bh, 0x00   ; Page number
    mov bl, 0x07   ; Attribute (light grey on black)
    int 0x10       ; Call BIOS
    ret

times 510-($-$$) db 0  ; Pad the bootloader to 510 bytes
dw 0xAA55              ; Boot signature

; com.fasm
use16

PORT equ 0x3F8  ; COM1 Base Port

; Initialize UART for serial communication
init_uart:
    pusha                      ; Save all general-purpose registers

    ; Disable all UART interrupts
    mov dx, PORT + 1           ; Interrupt Enable Register
    mov al, 0x00               ; Disable all interrupts
    out dx, al

    ; Set baud rate to 9600
    mov dx, PORT + 3           ; Line Control Register
    mov al, 0x80               ; Enable DLAB (Divisor Latch Access Bit)
    out dx, al
    mov ax, 0x000C             ; Divisor for 9600 baud rate (12)
    mov dx, PORT               ; Divisor latch low byte
    out dx, al
    mov dx, PORT + 1           ; Divisor latch high byte
    mov al, ah
    out dx, al

    ; Set line control register: 8 bits, no parity, 1 stop bit
    mov dx, PORT + 3
    mov al, 0x03               ; 8 bits, no parity, one stop bit
    out dx, al

    ; Enable FIFO, clear them, with 1-byte threshold (optional, for simplicity)
    ;mov dx, PORT + 2
    ;mov al, 0x01               ; Enable FIFO & set 1-byte threshold
    ;out dx, al

    popa                       ; Restore all general-purpose registers
    ret

; Send a byte to COM1
send_byte_to_com1:
    push ax
    push dx

    mov dx, PORT + 5           ; Line Status Register
wait_for_transmit_empty:
    in al, dx
    test al, 0x20              ; Wait for the transmitter to be empty
    jz wait_for_transmit_empty

    pop dx
    mov dx, PORT
    pop ax
    out dx, al
    ret

; Read a byte from COM1
read_byte_from_com1:
    push ax
    push dx

    mov dx, PORT + 5           ; Line Status Register
wait_for_data_ready:
    in al, dx
    test al, 0x01              ; Check if data is available
    jz wait_for_data_ready

    mov dx, PORT
    in al, dx
    pop dx
    pop ax
    ret

; Read a byte from COM1
read_byte_from_com1:
    ; push ax
    push dx

    mov dx, PORT + 5           ; Line Status Register
wait_for_data_ready:
    in al, dx
    test al, 0x01              ; Check if data is available
    jz wait_for_data_ready

    mov dx, PORT
    in al, dx                  ; Read the byte
    ; pop ax
    pop dx
    ret