Add drive letter assignment and the ability to change the drive.

src/SYSTEM.ASM

@@ -50,8 +50,9 @@ mov [0x82], ax
 mov [0x86], ax
 mov [0x8a], ax
 
-;Store the boot drive number
+;Store the boot drive number and set the drive letter
 mov [drive], dl
+call setdriveletter
 
 ;Print a welcome message
 mov si, welcomemsg
@@ -66,6 +67,10 @@ mov sp, 0x0
 sti
 
 ;Prompt for and read a command
+;Print the drive letter
+mov si, driveletter
+mov ah, 0x0
+int 0x21
 ;Print a prompt
 mov si, prompt
 mov ah, 0x0
@@ -76,10 +81,17 @@ mov al, 0x4e
 mov ah, 0x3
 int 0x21
 
-;Load an execute the program
+;Check for non-program stuff
 ;Check for an empty command
 cmp byte [input], 0x0
 jz shell
+;Check for a drive change command
+mov si, input + 1
+mov di, driveletter + 1
+call cmpstr
+jnc changedrive
+
+;Load an execute a program
 ;Load
 mov bx, 0x2000
 mov si, input
@@ -87,30 +99,138 @@ mov ah, 0x0
 int 0x22
 ;Check for errors
 cmp al, 0x1
-je error
+je cmderror
 ;Execute
 jmp 0x2000
 
-;Print an error message and return to the shell
-error:
+;Print a command error message and return to the shell
+cmderror:
 mov bh, 0x0
 mov ah, 0x3
 int 0x10
 dec dh
 mov ah, 0x2
 int 0x10
-mov si, errormsg
+mov si, cmderrormsg
 mov ah, 0x2
 int 0x21
 jmp shell
 
+;Change the drive
+changedrive:
+;Check which drive to change to
+cmp byte [input], "a"
+je .a
+cmp byte [input], "A"
+je .a
+cmp byte [input], "b"
+je .b
+cmp byte [input], "B"
+je .b
+cmp byte [input], "c"
+je .c
+cmp byte [input], "C"
+je .c
+cmp byte [input], "d"
+je .d
+cmp byte [input], "D"
+je .d
+;Print a drive error message and return to the shell
+mov si, driverrormsg
+mov ah, 0x2
+int 0x21
+jmp shell
+;Change
+.a:
+mov dl, 0x0
+mov byte [drive], dl
+call setdriveletter
+jmp shell
+.b:
+mov dl, 0x1
+mov byte [drive], dl
+call setdriveletter
+jmp shell
+.c:
+mov dl, 0x2
+mov byte [drive], dl
+call setdriveletter
+jmp shell
+.d:
+mov dl, 0x3
+mov byte [drive], dl
+call setdriveletter
+jmp shell
+
 ;Data
-drive      db 0x0
-welcomemsg db 0xd, 0xa, "Welcome to EttinOS!", 0xd, 0xa, 0x0
-prompt     db "> ", 0x0
-errormsg   db "Unknown command", 0x0
-input      times 0x4e db 0x0
-crlf       db 0xd, 0xa, 0x0
+drive        db 0x0
+driveletter  db "?:", 0x0
+welcomemsg   db 0xd, 0xa, "Welcome to EttinOS!", 0xd, 0xa, 0x0
+prompt       db "> ", 0x0
+driverrormsg db "Unknown drive", 0x0
+cmderrormsg  db "Unknown command", 0x0
+input        times 0x4e db 0x0
+crlf         db 0xd, 0xa, 0x0
+
+;Set the drive letter
+setdriveletter:
+;Check the drive number
+cmp dl, 0x0
+je .a
+cmp dl, 0x1
+je .b
+cmp dl, 0x2
+je .c
+cmp dl, 0x3
+je .d
+ret
+;Set
+.a:
+mov byte [driveletter], "A"
+ret
+.b:
+mov byte [driveletter], "B"
+ret
+.c:
+mov byte [driveletter], "C"
+ret
+.d:
+mov byte [driveletter], "D"
+ret
+
+;Compare two strings ending in a null at SI and DI and clear the carry flag if they are equal and set it if not
+cmpstr:
+;Store the initial registers in the stack
+push ax
+push bx
+;Compare the strings
+.loop:
+;Load the current characters
+mov al, [si]
+mov bl, [di]
+;Compare the characters
+cmp al, bl
+;Check for difference
+jne .neq
+;Check for the string end
+cmp al, 0x0
+je .eq
+;Repeat for the next characters
+inc si
+inc di
+jmp .loop
+;Clear the carry flag
+.eq:
+clc
+jmp .done
+;Set the carry flag
+.neq:
+stc
+.done:
+;Load the initial registers from the stack
+pop bx
+pop ax
+ret
 
 ;Print a CRLF
 printcrlf: