Contributor: MARTIN RICHARDSON
{
Here is my DRIVES routine again to return all valid drive letters on a
PC. This is a fix from the last version which incorrectly addressed
the local variables and wound up hosing memory. I also added some
extensive comments for readability. Enjoy! }
{*****************************************************************************
* Function ...... Drives
* Purpose ....... To return a string containing the valid drives for the
* current system.
* Parameters .... None
* Returns ....... A string of the valid drive letters.
* Notes ......... Rather than changing to each drive to see if it exists, we
* can instead call DOS Function 26h - Parse a file name.
* If the file name is invalid (eg, F:), then DOS will say
* so. So, by testing each drive letter as a file name,
* DOS will tell us which are good and which are bad!
* Author ........ Martin Richardson
* Date .......... August 6, 1993
* Update ........ 02-01-94: Corrected problem where local VAR variables were
* not being used, but a random memory location was
* instead!
* : Added comments for clarity.
*****************************************************************************}
FUNCTION Drives: STRING; ASSEMBLER;
VAR
DriveInfo: ARRAY[1..2] OF CHAR;
Buffer: ARRAY[1..40] OF CHAR;
DriveString: ARRAY[1..25] OF CHAR;
ASM
PUSH SI { Save Important Registers }
PUSH DI
PUSH ES
PUSH DS
MOV SI, SS { The Stack Segment (SS) points to the }
MOV DS, SI { VAR's above. Point DS to it... }
PUSH DS
POP ES { ...and ES as well. }
LEA SI, DriveInfo { DS:SI - Where we test each drive letter }
LEA DI, Buffer { ES:DI - FCB Buffer }
LEA BX, DriveString { DS:BX - Our resultant string }
MOV BYTE PTR [SI], '@' { The character before 'A' }
XOR CX, CX { Zero out CX }
@Scan:
INC BYTE PTR [SI] { Next Drive Letter }
MOV BYTE PTR [SI+1], ':'
MOV AX, 2906ドル { DOS Function 29h - Parse Filename }
INT 21h { DS:SI - String to be parsed }
{ ES:DI - FCB }
LEA SI, DriveInfo { DS:SI }
CMP AL, $FF { AL = FFh if function fails (invalid }
JE @NotValid { drive letter) }
INC CX { Add one more to our string length... }
PUSH CX { ...and save it. }
MOV CL, BYTE PTR DS:[SI] { Grab the valid drive letter... }
MOV [BX], CL { ...and stuff it into our result }
INC BX { Next position in result string }
POP CX { Get our length counter back }
@NotValid:
CMP BYTE PTR [SI], 'Z' { Did we go through all letters? }
JNE @Scan { Nope, so next letter }
LEA SI, DriveString { Store DriveString to #Result }
LES DI, @Result
INC DI
REP MOVSB
XCHG AX, DI { This is the only way to store the }
MOV DI, WORD PTR @Result { length that I can get to work. }
SUB AX, DI
DEC AX
STOSB
POP DS { Restore Important Registers }
POP ES
POP DI
POP SI
END;