Scor­pio News

  

April–June 1987 – Volume 1. Issue 2











Page 43 of 51











‘his may have been erating the obvious as T had already said, when referring to the example directory entty, that only one block ad been aliocated to the file. However, the concept of using zerot to show unallocated blocke ia fundamental to the vay directory entries relating to random files are interpreted I shall return to candom files shortly.

Let’s Finish looking at the example directory entry by examining the last four bytes of the top few. The firet of the four bytes 14 the extent byte. THs ahows ‘che highest extent number accessed by the directory entey In our Example, ‘the highest extent accersed is 0. Had the dicectory entry been full om 2 QDDS disk, the excene number would have been 3 since this directory eatey would contrel extents 0-2” A subsequent entry would have controlled extents 4-7 ud would have 7 in the extent byte if the entry wes full. Under CP/M 1 4, as Bleeady stated, the highest extent sumber allowed is 15 (16 extents undered 0- 15) giving # mazinun file size of 256K. Under CP/M 2.2 and CP/M 3 the maximum value allowed for the extent byte is 31 (32 extents) giving « maximum file size Of S12K... Hang on a biti Digital Research cold us that the maximum file sizes for Ce/H 2.2 and CP/M 3 are GHbytes and S2Mbytes respectively,

Under CP/M 2.2 or CR/M 3, when a file exceeds 512K, the $2 byte is used to hold the multiples of S12K and the extent byte starts numbering again from sero: This use of the 82 byte te pot st ail well documented. In fact 1 have searched Migh and Tow through the Digital Research manuals for references to the s2 byte and itr use io this context with little joy The result of this Inck of Information has led to the production of programs, both commercial (Wordstar) and Public Domain (D COM, $0 COM and DU COM) that cannot handle files lurger than 512K

‘The maximum value for the $2 byte under CP/M 22 is 15 (06 permutations numbered from 0.to 15) and 16 x 512K ir Obytes made up of 65536 records of 128 bytes each Under CP/M 3 ite marinum value ie 63 (64 permucations) and of course 64 x Si2k de 32Mbytes made up of 267164 records of 128 bytes each, Under CP/M 1-4 the 32 byte is unused in this context The 81 byte is unused by ail versions of CP/M up to 2 2 and is probably not used by CP/M 4, at least I haven’t yet found whore ituses (t, LF da fact ut doee

The Last byte of the four 4s che record count byte and {t shows how wany records are stored in the last extent written (Ie. the one indicated by ehe extent byte) If this byte has the value 0H then the extent ig full. In our example, one record has been written co extent 0. Had the extent number been 3, for Guample, then extents 0 2 would be assumed to be full. However, this isa’e necessarily teue when cefercing to candor files as we shall see now

With the block size chosen and having established that we are using 8 bit directory entries, as we have slready een, each directory entry controls up to Sik of s file th tact the dicectory entry Ls more than piece to stare Block numbers, it is Literally # map of the file

Directory entries for random Files

We cam split the 64K controlled by the entry up into 512 CP/M records of 128 bytes with chese records being numbered fron 0 to 511 Zach of the sixteen Uk block positions will contain 32\CP/M records and therefore the first block will contain records 0 to $2, the second 22 to 63 and eo on. If only record number 52 is weitten to file TEST AWD, ite directory entry will look like the one given below with only the second block position containing a block number

00544553 54202020 20524844 90000022 + TEST BND... .# ‘90200000 00000000 06000000 s0000000 + =. .

IE, on the other hand, only the 512th record had been written (record number ‘Siiy, only the Last block position would contain a block number,

The RC, EXT and 52 bytes will be set up as if the file had been written Sequentially, Le writing the last cecord to an extent will cause the RC byte tovread 80H,” ‘The only way that CP/M knows whether a block has been allocated or


This is an OCR’d version of the scanned page and likely contains recognition errors.











Page 43 of 51