Microdrive File Repair

(from Your Spectrum 8, October 1984)


This article must be read in conjunction with "Patching Up The 'Drives"


in issue 14.








RUNNING REPAIRS





Hands up those who've lost files on a Microdrive cartridge. Well, for


goodness sake, don't throw it away because Andrew Pennell's beginning


to solve the problem. Presented here is a way of examining and


printing up the suspect sectors ... and there's more to come.








Good as the ZX Microdrives are, like all forms of media, they're not


perfect. Every once in a while, faults occur that result in a 'File


not found' message and, of course, Murphy's Law determines that this


only happens to those that have not been backed-up.





The program I'm presenting is designed to enable 'repairs' to be made


to corrupted and unloadable files. It's in two parts. The first


(presented here) allows you to examine the cartridge for faults, and


print out all damaged and suspect sectors. The second, to be included


in a forthcoming issue, will allow individual sectors to be read in -


even if faulty - corrected, then written out, so you can still recover


the file. It won't be perfect, because badly corrupted files can be


impossible to fix; however, it'll work for many.








SAVE YOUR SECTORS





Before delving into the program, let's examine first why sectors


become unreadable. Usually, it's due to some mechanical or magnetic


abuse that results in some part of the tape losing bits of data. Thus,


when the Spectrum tries to read the affected sector, the data is


altered and the checksum saved with it no longer matches - so loading


fails to take place. What our section of machine code does is scan the


cartridge, reading each sector (whether corrupted or not) and storing


its particulars in a Basic array, z$. Given the sorted array, the


Basic part then uses the information to calculate which sectors are


damaged or missing altogether; the second stage uses this information


to allow access to individual sectors, in order to re-create them.





Our first move will be to enter the 500-odds bytes of machine code.


Those without an assembler will have to use the Hex loader given;


enter the code correctly, then save it on to cartridge with:





SAVE *"m";1;"SL.CODE" CODE 30000,500


Next, enter the main program, and save it with:


SAVE *"m";1;"repair" LINE 9000





Note that line 130 will only be accepted with the machine code


entered, and activated by RAND USR 30000.








CODE ANALYSIS





The code works by adding a command '*L' which scans a given cartridge,


storing its details in the array z$(200,13); then it sorts the data


using a bubble sort. NEWVEC is the additional syntax checker, which


okays the statement, gets the 'drive number, alters it to suit the


ROM, then does the actual work. Routine WATROM is similar to the one


detailed in All Change (see the August issue), altering the CALLs in


the program to suit whichever shadow ROM is in place. FIND is the main


entry point. It starts by creating an 'm' area in CHANS, and putting


the motor on; each sector is read in, and its checksum calculated to


see if it has corrupted. The ROM checksum routine cannot be used as it


alters the checksum byte - which makes it impractical for part two. If


the sector is used, its name, record number and sector number are


stored in z$, along with a flag that shows if it's an EOF sector - and


whether it's corrupted or not. The code at NEXT ensures the whole


cartridge has been read, before closing the 'm' channel. The border is


made green, and the sort routine entered.





SORT is a not very amazing bubble- sort routine. It sorts the elements


of z$ into order, using the crudest sort of algorithm possible. I


chose it for simplicity, not speed - though it is, of course, many


times faster than anything in Basic. The routine can take up to a


minute to sort a full cartridge; those feeling nervous are allowed to


break into it while it sorts.





Routine NXHDBF, the most important one of all gets down to the


business of scanning the tape, doing its checksum, and seeing if it's


used or not. CHKSUM is basically the same as the one in the ROM, but


with an instruction at the end removed. Finally, FINDZ$ is responsible


for searching the variables area for the array z$, and finding the


location of the first element. Note that no checks are made on the


dimensions or size of the array, only its existence. If z$ is not the


proper size, then Basic may crash - so beware.








ROUTINE ACTION





The business of examining the sector data is carried out in Basic -


because it's easier to change, and speed is not relevant. After the


'*L', each element of z$ contains 13 bytes of data: bytes one to 10


are the file-name, byte 11 the record number, byte 12 the sector, and


byte 13 the flag. Option 1 prints all the file names, like CAT but


including CHR$ 0 file names. While using it, you may get strange file


names at the top of the catalogue; don't worry - all cartridges have a


couple of strangely-named sectors on them (as a by-product of the


FORMAT routine) all starting with CHR$ 0. Option 2 prints a sector


list, which consists of each used sector, its file name record number,


sector number, and type. From this, you can work out what's missing


from it, as record numbers should rise from zero up to one with EOF


against it. It also tells you if any are corrupted, though you don't


have to scan lines of information to find the faults; Option 3 prints


all the corrupted sectors, while Option 4 will examine all the sectors


of a given file and tell you if any are missing or corrupted. As it's


in Basic, you can change it to suit your needs.





All this allows you to find the faults in your cartridges; watch out


for part two where you'll discover how to fix them.