Ensure first argument is filename

edlin.asm

@@ -1,7 +1,13 @@
 cpu 8086
 org 0x100
 
-db 0xEB, 0x7F ; 0100 jmp short 0x181
+; FCB defines
+fcb_filename equ 1
+
+; PSP defines
+psp_fcb_1 equ 0x005c
+
+jmp entrypoint
 
 __0102:
 	db 13, 10, "The IBM Personal Computer EDITOR", 13, 10
@@ -11,17 +17,26 @@ __0102:
 __0150 db "Licensed Material - Program Property of IBM"
 
 
-db 0xBA, 0x5C, 0x09 ; 017B mov dx,0x95c
-db 0xE9, 0xFB, 0x02 ; 017E jmp 0x47c
-db 0xC6, 0x06, 0x7F, 0x0A, 0x00 ; 0181 mov byte [0xa7f],0x0
-db 0xBC, 0x48, 0x0D ; 0186 mov sp,0xd48
-db 0x82 ; 0189 db 0x82
-db 0x3E, 0x5D ; 018A ds pop bp
-db 0x00, 0x20 ; 018C add [bx+si],ah
-db 0x74, 0xEB ; 018E jz 0x17b
-db 0x0A, 0xC0 ; 0190 or al,al
-db 0xBA, 0x41, 0x09 ; 0192 mov dx,0x941
-db 0x75, 0xE7 ; 0195 jnz 0x17e
+print_filename_missing_error: ; 017b
+	mov dx, filename_missing_error
+
+_trampoline_error_1:
+	jmp error
+
+entrypoint: ; 0181
+	mov byte [0x0a7f], 0
+	mov sp, 0x0d48
+
+ensure_file_argument:
+	db 0x82, 0x3e ; cmp byte […], byte +… (extended)
+	dw psp_fcb_1 + fcb_filename
+	db ' '
+	je print_filename_missing_error
+
+	db 0x0a, 0xc0 ; or al, al (longer encoding)
+	mov dx, invalid_name_error
+	jnz _trampoline_error_1
+
 db 0xBE, 0x3E, 0x09 ; 0197 mov si,0x93e
 db 0xBF, 0x65, 0x00 ; 019A mov di,0x65
 db 0xB9, 0x03, 0x00 ; 019D mov cx,0x3
@@ -324,9 +339,12 @@ db 0xC3 ; 0474 ret
 db 0xB4, 0x10 ; 0475 mov ah,0x10
 db 0xCD, 0x21 ; 0477 int 0x21
 db 0xBA, 0xB9, 0x09 ; 0479 mov dx,0x9b9
-db 0xB4, 0x09 ; 047C mov ah,0x9
-db 0xCD, 0x21 ; 047E int 0x21
-db 0xCD, 0x20 ; 0480 int 0x20
+
+error: ; 047c
+	mov ah, 9
+	int 0x21
+	int 0x20
+
 db 0x8B, 0x16, 0x90, 0x0A ; 0482 mov dx,[0xa90]
 db 0x8B, 0x3E, 0x92, 0x0A ; 0486 mov di,[0xa92]
 db 0x3B, 0xDA ; 048A cmp bx,dx