CPU 8086
ORG 0x3000

;List the files on a drive

;Store the drive number
mov [drive], dl

;Check for an empty tail
cmp byte [si], 0x0
je start

;Change the drive if needed
;Check for a drive specification and print an error message and abort if
;the tail is not one
cmp byte [si + 0x1], ":"
jne error
cmp byte [si + 0x2], 0x0
je changedrive
cmp byte [si + 0x2], 0x20
je changedrive
jmp error
;Check which drive to change to
changedrive:
cmp byte [si], "a"
je cha
cmp byte [si], "A"
je cha
cmp byte [si], "b"
je chb
cmp byte [si], "B"
je chb
cmp byte [si], "c"
je chc
cmp byte [si], "C"
je chc
cmp byte [si], "d"
je chd
cmp byte [si], "D"
je chd
;Print an error message and abort if the drive is unknown
jmp error
;Change
cha:
mov dl, 0x0
mov byte [drive], dl
jmp start
chb:
mov dl, 0x1
mov byte [drive], dl
jmp start
chc:
mov dl, 0x2
mov byte [drive], dl
jmp start
chd:
mov dl, 0x3
mov byte [drive], dl

start:

;Load the disk description table
;Set the source
mov dl, [drive]
mov ch, 0x0
mov dh, 0x0
mov cl, 0x1
;Set the destination
mov si, buffer
mov bx, si
;Set the size
mov al, 0x1
;Load
mov ah, 0x2
int 0x13

;Store the disk values used for the rest of the call
mov ax, word [buffer + 0xb]
mov word [sectorsize], ax
mov ax, word [buffer + 0xe]
mov word [bootsectors], ax
mov al, byte [buffer + 0x10]
mov byte [fats], al
mov ax, word [buffer + 0x11]
mov word [rootentries], ax
mov ax, word [buffer + 0x16]
mov word [sectorsperfat], ax
mov ax, word [buffer + 0x18]
mov word [sectorspertrack], ax
mov ax, word [buffer + 0x1a]
mov word [sides], ax

;Load the root
;Set the source
mov ah, 0x0
mov al, [fats]
mul word [sectorsperfat]
add ax, [bootsectors]
push ax
call calcsource
;Set the destination
mov si, buffer
mov bx, si
;Set the size
push dx
mov ax, [rootentries]
mov dx, 0x20
mul dx
mov dx, 0x0
div word [sectorsize]
pop dx
push ax
;Load
mov ah, 0x2
int 0x13

;List
;Set SI to the root
mov si, buffer
;Initialise the loop
mov cx, word [rootentries]
mov ax, 0x0
loop:
;Store CX in the stack
push cx
;Check for an empty entry
cmp byte [si], 0xe5
je skip
;Check for the end of entries
cmp byte [si], 0x0
je done
;store AX in the stack
push ax
;Print the main part of the name
mov cx, 0x8
push ax
printname:
mov al, [si]
mov ah, 0xe
int 0x10
inc si
loop printname
;Print a space
mov al, 0x20
mov ah, 0xe
int 0x10
;Print the extension
mov cx, 0x3
printext:
mov al, [si]
mov ah, 0xe
int 0x10
inc si
loop printext
;Print a newline
mov al, 0xd
mov ah, 0xe
int 0x10
mov al, 0xa
mov ah, 0xe
int 0x10
;Load AX from the stack
pop ax
skip:
;Set SI to the next entry
add ax, 0x20
mov si, buffer
add si, ax
;Load CX from the stack
pop cx
loop loop

done:
int 0x20

error:
mov si, driverrormsg
mov ah, 0x2
int 0x21
int 0x20

;Data
drive           db 0x0
sectorsize      dw 0x0 ;bytes
bootsectors     dw 0x0
fats            db 0x0
rootentries     dw 0x0
sectorsperfat   dw 0x0
sectorspertrack dw 0x0
sides           dw 0x0
driverrormsg    db "Drive not found", 0x0

;Calculate the source arguments for loading data from the disk
calcsource:
push ax
push bx
mov bx, ax
mov dx, 0x0
div word [sectorspertrack]
add dl, 0x1
mov cl, dl
mov ax, bx
mov dx, 0x0
div word [sectorspertrack]
mov dx, 0x0
div word [sides]
mov dh, dl
mov ch, al
pop bx
pop ax
mov dl, byte [drive]
ret

;Buffer
buffer:
 