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Note the 'token' is really the character codes of the keyword.
DS$
Get the most recent error/status code from a "disk" device.
This reserved variable returns the diskStatus_string which has been (or will now be) read from the error/status channel (15) corresponding to device "DosFA" (a secret variable). Unfortunately BASIC provides no direct way to set "DosFA", and attempting to supply any parameter to DS$ (such as a device number) will generate SYNTAX ERROR. Otherwise, if a diskStatus_string already exists, then DS$ can not generate an error. If the diskStatus_string has been erased (see below), then DS$ will now read a new diskStatus_string from channel 15 of the device specified by the secret "DosFA". This might cause an I/O error, like DEVICE NOT PRESENT (for example, if the device number was changed, or the device has been powered-off or disconnected). If you have previously OPEN'ed a file with fileNumber of zero to any device (I believe only the keyboard is allowed) then you may get either FILE OPEN ERROR or in some cases (not sure of how each are triggered) the cursor will appear on screen and wait for you to enter some text (which will be stored as the diskStatus_string, and your fileNumber 0 will be CLOSE'd). See examples. Assuming you don't have an OPEN file 0, DS$ works fine in conjunction with disk-based commands like DIRECTORY, DLOAD, and such. But because the secret "DosFA" is not updated by other commands that are often used with a disk, like CLOSE, CMD, GET#, INPUT#, LOAD, OPEN, PRINT#, SAVE, and VERIFY, using DS$ may report on the wrong device. This is especially true in a program that uses multiple "disk" devices at once (any device number 8 to 11). To work around this bug, you can POKE the DosFA or use the "BASIC 2 Method" of reading the diskStatus_string (that is, INPUT #15,E,M$,T,S where '15' is an already OPEN'ed file to channel 15 of the desired device and M$ would be the diskStatus). If that sounds a bit archaiac / difficult to understand, that's because it is! Hence the reason for including DS and DS$ in BASIC... too bad it is buggy. Here is the location of the "DosFA" secret variable for some machines:
*Update* I just thought (and performed some testing) of a less machine-specific way to work around the bug! Use DCLOSE with a file# not in use, and specify the desired device as a parameter. For example, to be sure the next read of DS$ will refer to device 9, use this just before: DCLOSE #77, U9 (where 77 is a file# that was never opened). The only "problem" is it only works with the same devices that all disk commands accept (only device 8 to 11). Using the POKE method works with any IEEE/IEC device (4 to 30). Reading DS$ does not clear the exisitng diskStatus_string, so you may read it multiple times (if diskStatus_string did not exist, then ST will be updated when the string is read from the device). The diskStatus_string will remain until you use any I/O command that references a device number between 8 and 11 (traditionally disk drives). This includes both "disk commands" from BASIC 3.5+ and generic I/O commands of v2.0; all of them clear DS$ (i.e., the diskStatus_string), and a few of the disk commands will read a new diskStatus_string. See examples. Like all BASIC variables, only the first two characters of the name are significant. So you may also use DSTAT$ if you prefer (note the "TAT" characters are superfluous). Like most BASIC reserved variables, attempting to assign a value to it will generate SYNTAX ERROR. Warning! On the C128, using the built-in MONITOR will destroy the descriptor for the diskStatus_string, even if you don't use any MONITOR disk-commands. Thus when you return to BASIC and try to access DS or DS$ you will get unreliable results until you use a disk command or perform CLR (directly or indirectly). Examples (normal):
Examples (with OPEN fileNumber of zero):
CLR, DS; APPEND, BACKUP, BLOAD, BOOT, BSAVE, CATALOG, COLLECT, CONCAT, COPY, DCLEAR, DCLOSE, DIRECTORY, DLOAD, DOPEN, DSAVE, DVERIFY, HEADER, RECORD, RENAME, RUN, SCRATCH © H2Obsession, 2014 |