Date   : Thu, 09 Apr 1981002:35:00-MST
From   : W8SDZ@MIT-MC (Keith B. Petersen)
Subject: CP/M random write buffer flush

My appologies to those who get this message twice.  I thought
it important enough to pass along to as many people as possible.

If your CBIOS uses any kind of deblocking to interface between
CP/M's 128-byte sectors and your disk's actual physical sector
size, the following patch MUST be done to your system.  The
addresses shown are for standard distribution CP/M.  Those who
have Morrow systems will find the patch area at 23D2 instead of
the 1CD2 shown.  Early issues of Morrow's CP/M 2.2 did not have
this patch.  After you make the patch, if you are using Morrow,
you can delete the "CALL FLUSH" statements in your console input
and output routines, thus cutting down on the amount of disk
accesses when running WordStar or other similar text editors that
scroll through a disk file.

DIGITAL RESEARCH CP/M [R] 2.2 FIELD SOFTWARE CHANGE
         Copyright (c) 1980   Digital Research

ID# CPM22-0001    PROGRAM: BDOS     ISSUE DATE: 02/19/80


ERROR DESCRIPTION:  The following change affects only those CP/M
systems which are using the optional blocking and deblocking al-
gorithms listed in Appendix G of the CP/M Alteration Guide.  If
you are in doubt as to the applicability of this field change,
please contact Digital Research or your CP/M distributor.

PATCH PROCEDURE:  Ensure you have an archive copy of the distri-
buted MOVCPM.COM file.  Make changes to a version of MOVCPM>COM
by carefully following the steps shown below:  MOVCPM.COM is
loaded into memory using DDT and the changes are made using the
Assemble (A) and Set (S) commands.  After making the changes,
return to the console command processor using the Go (G) command,
and SAVE the altered memory image on disk.  The memory image on
tracks 0 and 1 must also be updated.  This can be accomplished
by executing the new MOVCPM program, as described in the CP/M
Alteration Guide, and integrating your customized I/O system.


                     ddt movcpm.com
                     DDT VERS 2.0
                     NEXT PC 
                     2700 0100
                     -a1cd2
                     1CD2 nop
                     1CD3 nop
                     1CD4 lxi h,0
                     1CD7


                     G0
  
                     save 38 movcpm.com


NOTE:  This Field Software change is not installed in the CP/M
version 2.2.  It must be installed in all systems which use the
deblocking algorithms listed in Appendix G of the CP/M Alteration
Guide.

END OF FIELD CHANGE NOTICE.

The code you are replacing is:
       DCR     C
       DCR     C
       JNZ     (ADDRESS)

The patch effectively nullifies that code, causing CP/M to properly
instruct your CBIOS to update the last sector written.