fat: add fread

fat.asm

@@ -106,6 +106,7 @@ read:	push ax
 	mov dx, word [cs:seek.dl]
 
 	int 0x13
+	int 3
 
 	pop dx
 	pop cx
@@ -196,17 +197,18 @@ scadir:
 .name:	db "HELLO   "
 .ext:	db "COM"
 
-open:
+fopen:
 	push ax
 	push cx
 	push dx
 	push bx
 	push ds
 	push es
+	cmp word [cclus], 0
+	jne .err
 	xor ax, ax
 	mov ds, ax
 	mov es, ax
-	mov [csec], ax
 	call dirsec
 	call dirlen
 .loop:	xor dx, dx
@@ -217,10 +219,11 @@ open:
 	jnc .found
 	inc ax
 	loop .loop
-	stc
-.found:
-	mov [cclus], bx
-	pop es
+.err:	stc
+	jmp .ret
+.found:	mov [cclus], bx
+	mov word [csec], 0
+.ret:	pop es
 	pop ds
 	pop bx
 	pop dx
@@ -228,11 +231,43 @@ open:
 	pop ax
 	ret
 
+fread:  push ax
+	push cx
+	push dx
+	cmp word [cs:cclus], 0
+	je .err
+	call clusec
+	push ax
+	mov ax, [cs:cclus]
+	sub ax, 2
+	mov cl, [cs:fdc.sc]
+	xor ch, ch
+	mul word cx
+	; add current cluster offset, cluster start and current sector
+	pop cx
+	add cx, [cs:csec]
+	add ax, cx
+	adc dx, 0
+	; dx:ax now point to the sector that should be read next
+	call seek
+	call read
+	jmp .ret
+.err:	stc
+.ret:	pop dx
+	pop cx
+	pop ax
+	ret
+
 main:	mov [seek.dl], dl ; save drive number
 
-	int 3
-	call open
-	int 3
+	call fopen
+	mov bx, 0x1100
+	call fread
+
+	mov ax, 0x0100
+	mov ds, ax
+	mov es, ax
+	call 0x100:0x100
 
 .hlt:	hlt
 	jmp .hlt