Scor­pio News


April–June 1987, Volume 1, Issue 2

Page 44 of 51

not {a by whether there {8 a zero in the block position of the directory entry. Wha CP/M doesn’t know is which records have been written to the block; this ie Left for the programmer to determin

one vay of doing this {s to use the write random with zero 111 function when weiting to 4 previously unallocated block. To establish, from within « progran, hatnge or not’a bigck has bean allocated to’s file ta to read the desired Fecord using the read rand trying to read a block tha 7 Felevant position in the directory entry. ith sero fill, If no error code i Tocated which could contain the. wanted record. If you previously weed ro Fil function when writing « record to this block, examination of the contents of the cequired record will reveal zeros 1f nothing has been written to Stor data 18 something hav been written.

A point vorth noting is that the CP/M DIR command will only show a file if aurectory entry exitte for extent 0. For thie reason, writing one Fecord to Fecord position 63535, for example, will result in two directory entries being Greaved; an empty one covering extent 0 and one covering the 128th record of extent 512

Another observation about random files {a the vay that PIP.COM handles them (or more accurately, it doesn’t). PIP,COM reade and writes files sequentially and Will thecefore get an end of file code from the BDOS when it encounters an Unallocated block ina file. Normally, this vould be adequate ae most files indeed end thie vay If faced with the example of a file that contains only record 65535, PIP.COM will only copy the empty directory entry for extent ©

BIOS date structures

tne dlcectory Questions in detail, some information about BIOS dace structures 18 required.

The disk routines in the BX

contain data areas and buffer epace that will be used by the BDOS when it needs to have information that is system dependent. These areas ave known as the Disk Parameter Headere (DPil’s), Disk Parameter Blocks “(DPB’s}, Checksum Vectors and Allocation Vectors We’ll Look at these in the next. issue.

MAP 80 Multl-Purpose Interface (MPI) Review by P.D. Coker

Theater of 0-BUS compatible products

controllers – initially by the G then the GH829 with SAst Suppose Tor a Winchester, ciesely followed by the even more’ supportive CMoCd Yap 80’s VEC vas also capable of controlling 5.25" drives and Nescon/Lucat. also produced an FDC card. So why have Map produced yet another card?

been quite well served for floppy ,,

The MPI is more than Just an PDC, catering for the usual range of disk size with support fore Winchester drive or two. By changing a few Linke and the Foc ‘chip one can convert co Nascom FOC compatibility rather then the more usual Gensni Map vacandard- Ip aleo nes a CTC and two channels of S10, one of which Seva teandacd 2230 and the other Conforms to the proposed standasd for KECes (ign speed aulti-deop incerface)-. The ETC can Yo used to generate soteware Selfccable baud cates or, if the user, wishes, on-board crystals can be ue Generate two frequencies’ on the HPI board which are independent of the syst Clock. Ie une 26 porte and, in ay experience, can substitute for either of the tvo earlier Gemini “OCs – T’have po experience of the mort recent Gemini FDC.

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

Page 44 of 51