Commodore Plus 4 - Page 362

380 Using Peripheral Devices
290 Inform user that the file was not found and quit.
300 Inform user that the file was found.
310 Now, B points to the file type byte for the current file entry.
320 If the file is not scratched, inform the user and quit.
330-340 Determine the desired file type.
350 If not a valid input, repeat the question.
360 Final chance to bail out without affecting the disk.
370 If it is not a definite yes, quit.
380 Move the disk buffer pointer to the file type byte for the current file
entry.
390 Put the new file type byte into the disk buffer.
400 Write the disk buffer out to the disk.
410 Check for disk errors.
420 Print message to user.
430 Perform a validate function on the disk to update the BAM.
440 Check for disk errors.
450 Close the files and stop processing.
460 If no disk error, return.
470 Print out the error message, close the files, and quit.
Using a Disk Drive with Machine Language
All of the direct-access programming techniques described in the preceding
BASIC section are available in machine language. The function of the BASIC
commands described to open and use the command and direct-access channels
can be accomplished in machine code with the appropriate operating system
subroutines.
In general terms, you can choose from three levels of communication with the
disk drive. At the highest level are the save and load routines that perform all the
necessary functions to save or load a section of RAM from disk. The middle level
allows the maintenance of multiple open files, similar to OPEN commands in
BASIC. At the most fundamental level, it is possible to listen and talk to the disk
drive directly through the serial bus subroutines. Each of these levels is explored
and example programs are given in this section. The complete descriptions of the
operating system subroutines used can be found in Chapter 5.