N CHKDRV.COM
A 100
OR   AL,AL   ;AL=0 if 1st drive valid
JZ   010A       ; or FF if invalid
MOV  AL,02      ;Return errorlevel=2
MOV  AH,4C      ;Terminate
INT  21         ; thru DOS
MOV  DL,[005C]  ;Get drive number+1
DEC  DL         ; subtract 1
JNS  0116       ; not negative if okay
MOV  AL,03      ;Return errorlevel=3
JMP  0106       ; exit
MOV  AH,04      ;Verify diskette fn
MOV  AL,01      ; 1 sector
MOV  CH,00      ; track 0
MOV  CL,01      ; sector 1
MOV  DH,00      ; side 0
INT  13         ; thru BIOS
JNC  012C       ;NoCarry=okay
MOV  AH,00      ;Reset diskette
INT  13         ; thru BIOS
MOV  AL,01      ;Return errorlevel=1
JMP  0106       ; exit
MOV  AL,00      ;Return errorlevel=1
JMP  0106       ; exit

RCX
30
W
Q