Commodore Plus 4 - Page 359

The Disk Drive 347
Line-by-Line Explanation
10 Open the command channel.
20 Check for error. If an error is present, exit.
30 Construct a string containing the machine-language subroutine.
40 Write the subroutine into the 1541's memory at $0500.
50 Execute the machine-language subroutine.
60 Retrieve the saved count from the 1541's memory.
70 Get the count.
80 Print it out.
90 Close the command channel.
100 The data are this machine code routine:
$0500 LDY #$00 Count of active buffers found.
$0502 LDX #$06 Pointer to which buffer checking.
$0504 LDA $A7,X Get channel number of this buffer.
$0506 CMP #$FF Compare to inactive buffer value.
$0508 BEQ $050B If equal, skip next instruction.
$050A INY Increment the count.
$050B DEX Decrement the pointer.
$0500 BPL $0504 If not done, go back to get next the channel
number.
$050E STY" $1B Store count for BASIC program's retrieval.
$0510 RTS Return.
UnSCRATCHing a Disk File
When the disk drive is instructed to SCRATCH a file, it does not actually remove
the file's contents from the diskette. Instead, it changes a byte in the diskette's
directory to indicate that the file was SCRATCHed and deallocates all of the file's
sectors in the BAM.
If you discover that you have SCRATCHed an important file for which you
have no backup, you can sometimes recover the file. It is important to note that
this procedure is for EMERGENCIES only. This is NOT a recommended
procedure. It is far, far better to keep plenty of backup copies of all of your work.
If you must try it, please note the following:
1. Your chances of success are MUCH better if NOTHING has been written to
the diskette since the SCRATCH occurred. Do not try this procedure if you
performed disk write operations after the SCRATCH.