Scor­pio News


July–September 1987 – Volume 1. Issue 3

Page 14 of 67

and the drive cos Foutines (in variable @cdrv).

Similarly, when the READ or WRITE entry pointe in the BIOS Jump table are called, “these coltines uae the stored code Zor the current drive to index ato the drive table and fetch the addrees of the appropriate XDPH, Bear in mind that any ercors due to there being no physical drive present should have already been found by the SELDSK routine.

once armed with the address of the correct XDPH, the SELDSK, READ and WRITE outines use it eo extract any other infor: A detasied a

The NTT {1014 provides the with the drive by the Eating cold boot. other shan

perhaps, during error recovery.

‘The READ and WRITE field ds provide the elected drive and ai

set the track and sector numbers, set the DMA ‘and indicated which memory bank is to be used in the coming disk transfer. All that de required of tne BIOS in ite simplest form is to select the required memory bank and transfer data to or from one physical disk sector.

A factiity existe within GP/M 3 for the BDOS to instruct the BIOS to

fer multipte sequential sectors cos contiguous area of RAM, ‘starting st

the DHA address. The BboS tells the BIOS how many sectore to read’ or weite and

the Br0s vill, Af thts facility Tenented, continue transfercing

ore to of from disk until th ‘been copied: in case

ty the writer of the BIOS,” the 3005 will

DHA address and will call. the 3103

Te is up co the BIOS Co Sgnore the surplus celue EE Ge has alvendy teanafected the required amount of dat,

The LOGIN field de used by SELDSK to find the address of the routine appropriate to the disk drive being aelected.

The UNIT £1014 Le « single dyte chat holds the FOC relative drive code, Le. the sctual value that fe output to the disk controller to select the Fequired drive for use. The BIOS extracts thie code when any of the SELDSK, READ or WRITE routines is called.

The TYPE entry of the XDPH Je a ecratchpad location for use by the BIOS. weeh recommend that At is used to density in systems that support

The CSV entry und checksums contained in the Snstead of adding together all. 228 by gub-totals for each of the four 32 byte ry four tub-totals are then exclusive ORed together to produce the checksum byte. This KORing together reduces the number of bits lost due to overflow (due to adding as for CP/M 2.2) and therefore each checksum byte contains more information about the directory record, For this reason, the likelihood of Adentical checksums being returned for directory records on different disks is reduced and hence the possibility of a disk change being missed is aleo reduced.

identical to that of CP/M 2.2 but the lated ina slightly different manne:

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

Page 14 of 67