Commodore Plus 4 - Page 336

384 Using Peripheral Devices
begin with the letter F. The second wild card is the question mark, which is used to
mean any character in that specific position matches. For example, the use of
"F??PRG" as a file name matches all file names on the diskette starting with F,
followed by two characters, and ending in PRG. The key difference between the
two wild cards is that the asterisk matches all combinations of any number of
characters and must be at the end of the string, whereas the question mark
matches any single character and can be in any position in the string.
The information in this section has been verified with a 1541 disk drive. The
BASIC and machine-language access to the DMA disk should be the same.
However, the commands that the drive itself recognizes may be different. Please
refer to the manual for specific information on the DMA disk.
Using a Disk Drive with BASIC
The disk drive can be used to save BASIC programs or to create and manage data
files through BASIC. There are different types of files that can be used for
information storage, or a program can access specific areas on the disk directly.
Each type of file and all the direct-access commands are covered in this section.
A number of maintenance functions are available for use with diskettes. These
are detailed in Chapter 1 and reviewed in this chapter (see Diskette Mainte
nance). In particular, a new diskette must be formatted before its first use.
If the red light on the disk drive blinks at any time, an error has occurred. To
read the error, type
PRINT DS$
For an explanation of DOS errors, see Appendix A. A program can check for
DOS errors by looking at the reserved variable DS. If DS is not zero, then an
error has occurred and DS gives the error number. The error message can then be
printed by printing DS$. Error checking should be done frequently in BASIC
programs that use the disk. Most errors in disk 1/ O are reported in DS and DS$.
A notable exception is the end-of-file that is returned in the status variable, ST
with bit 6 set (a decimal value of 64).
Saving, Loading, and Verifying Programs
Since one of the main uses for a disk drive is to save and retrieve BASIC
programs, BASIC has built-in commands to make it easy. You can save programs
with the DSAVE (or the SAVE) command.
DSAVE filename9I>drive,TJvjiit