kernel.asm

   mov ax, 0x07C0  ; set up segments
   mov ds, ax
   mov es, ax

   mov bh, 0
   mov ah, 0x2
   int 0x10
   mov cx, 2000 ; print 2000 chars
   mov bh, 0
   mov bl, 0x1E ; green bg/blue fg
   mov al, 0x20 ; blank char
   mov ah, 0x9
   int 0x10

   mov si, welcome
   call print_string

 mainloop:
   mov si, prompt
   call print_string

   mov di, buffer
   call get_string

   mov si, buffer
   cmp byte [si], 0  ; blank line?
   je mainloop       ; yes, ignore it

   mov si, buffer
   mov di, cmd_hi  ; "hi" command
   call strcmp
   jc .helloworld

   mov si, buffer
   mov di, cmd_help  ; "help" command
   call strcmp
   jc .help

   mov si, buffer
   mov di, cmd_shutdown  ; "shutdown" command
   call strcmp
   jc .shutdown

   mov si,badcommand
   call print_string
   jmp mainloop

 .helloworld:
   mov si, msg_helloworld
   call print_string

   jmp mainloop

 .help:
   mov si, msg_help
   call print_string

   jmp mainloop

 .shutdown:
   mov ax, 0x1000
   mov ax, ss
   mov sp, 0xf000
   mov ax, 0x5307
   mov bx, 1
   mov cx, 3
   int 0x15

 welcome db 'Loading Plutonium Kernel (Plutonium version pm.dev-b0.1.1)...', 0x0D, 0x0A, 0
 msg_helloworld db 'pm.dev-b0.1.1, 4-9-15', 0x0D, 0x0A, 0
 badcommand db 'That command is not recognized as a system operation.', 0x0D, 0x0A, 0
 prompt db 'Plutonium >', 0
 cmd_hi db 'info', 0
 cmd_help db 'help', 0
 cmd_shutdown db 'shutdown', 0
 msg_help db 'My OS: Commands: hi, help', 0x0D, 0x0A, 0
 buffer times 64 db 0

 ; ================
 ; calls start here
 ; ================

 print_string:
   lodsb        ; grab a byte from SI

   or al, al  ; logical or AL by itself
   jz .done   ; if the result is zero, get out

   mov ah, 0x0E
   int 0x10      ; otherwise, print out the character!

   jmp print_string

 .done:
   ret

 get_string:
   xor cl, cl

 .loop:
   mov ah, 0
   int 0x16   ; wait for keypress

   cmp al, 0x08    ; backspace pressed?
   je .backspace   ; yes, handle it

   cmp al, 0x0D  ; enter pressed?
   je .done      ; yes, we're done

   cmp cl, 0x3F  ; 63 chars inputted?
   je .loop      ; yes, only let in backspace and enter

   mov ah, 0x0E
   int 0x10      ; print out character

   stosb  ; put character in buffer
   inc cl
   jmp .loop

 .backspace:
   cmp cl, 0    ; beginning of string?
   je .loop ; yes, ignore the key

   dec di
   mov byte [di], 0 ; delete character
   dec cl       ; decrement counter as well

   mov ah, 0x0E
   mov al, 0x08
   int 10h      ; backspace on the screen

   mov al, ' '
   int 10h      ; blank character out

   mov al, 0x08
   int 10h      ; backspace again

   jmp .loop    ; go to the main loop

 .done:
   mov al, 0    ; null terminator
   stosb

   mov ah, 0x0E
   mov al, 0x0D
   int 0x10
   mov al, 0x0A
   int 0x10     ; newline

   ret

 strcmp:
 .loop:
   mov al, [si]   ; grab a byte from SI
   mov bl, [di]   ; grab a byte from DI
   cmp al, bl     ; are they equal?
   jne .notequal  ; nope, we're done.

   cmp al, 0  ; are both bytes (they were equal before) null?
   je .done   ; yes, we're done.

   inc di     ; increment DI
   inc si     ; increment SI
   jmp .loop  ; loop!

 .notequal:
   clc  ; not equal, clear the carry flag
   ret

 .done:
   stc  ; equal, set the carry flag
   ret

   dw 0AA55h ; some BIOSes require this signature