Several parameter bytes in fixed locations in the BIOS can be patched
busing DDT and written back to disk using SYSGEN. When a MOVCPM or
SYSGEN image is loaded into memory using DDT, the parameter bytes are
at the following locations:

   2133h - MODE byte (see MODE byte below)
   2134h - Default IOBYTE (see IOBYTE below)

   2135h - Console input mask (input byte AND'd with this mask) 
   2136h - Reader input mask (input byte AND'd with this mask)
   2137h - Console output mask (output byte AND'd with this mask)
   2138h - Punch output mask (output byte AND'd with this mask)
   2139h - List output mask (output byte AND'd with this mask)

   213Ah - Motor startup delay in 0.1 sec intervals
   	   Try 4 (0.4sec) for 3.5" drives and 8 (0.8sec) for 5.25"

   213Bh - Head settle delay after final step (in ms)
   	   25ms is a safe generic value

   213Ch - Delay before reversing step direction (in ms)
   	   25ms is a safe generic value

   213Dh - Head settle delay after head load (in ms)
   	   Use zero for 3.5" drives or 5.25" drives which don't have
   	   a head load solenoid. Use 50ms if you hear a head load
   	   solenoid click when a drive is selected.


----------------------------------------------------------------------
MODE byte flags at 2133h in SYSGEN image

   fCLDCMD in 01h bit, true = CCP process cmd on cold start
   fWRMCMD in 02h bit, true = CCP process cmd on warm start
   fENAINT in 10h bit, true = re-enable interrupts after disk I/O

   The default MODE byte has the 10h bit set so interrupts are enabled
   when exiting disk I/O calls

   To use CP/M command execution on cold and/or warm start, set the
   corresponding flag(s) true and place the command string into the CCP
   input buffer at 0B08h in the SYSGEN image in memory. The length of 
   the string should be placed at 0B07h.


----------------------------------------------------------------------
IOBYTE at 2134h in SYSGEN image

    CON device (bits 1,0):
	00 - TTY serial port at A0h*
	01 - CRT serial port at F0h
	10 - BAT indirect through RDR/LST
	11 - UC1 serial port at 90h

    RDR device (bits 3,2): 
	00 - TTY serial port at A0h
	01 - PTR serial port at F0h*
	10 - UR1 serial port at 90h
	11 - UR2 serial port at A0h

    PUN device (bits 5,4):
	00 - TTY serial port at A0h
	01 - PTP serial port at F0h*
	10 - UP1 serial port at 90h
	11 - UP2 serial port at A0h

    LST device bits (7,6):
	00 - TTY serial port at A0h
	01 - CRT serial port at F0h
	10 - LPT parallel port at E0h*
	11 - UL1 serial port at 90h

    * Default setting (94h)


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Saving a SYSGEN Image to Disk (to modify with DDT)

In addition to writing a CP/M boot image to the boot tracks on disk,
SYSGEN also allows reading the CP/M boot image into memory. Following
is a typical session using SYSGEN to read the CP/M image from the boot
tracks and save it as a standard CP/M file on disk:

   A>SYSGEN
   MFA SYSGEN VER 2.0
   SOURCE DRIVE NAME (OR RETURN TO SKIP)A
   SOURCE ON A, THEN TYPE RETURN
   FUNCTION COMPLETE
   DESTINATION DRIVE NAME (OR RETURN TO REBOOT)

   A>SAVE 38 CPMOLD.COM

Here is a sample session using DDT to load the saved SYSGEN image and
display the parameter locations for modification using the DDT "S"et
command. DDT exit is by CTRL-C or G0 (GO zero).

   A>DDT CPMOLD.COM
   DDT VERS 2.2
   NEXT  PC
   2700 0100
   -S2133
   2133 10 00	(type in new value or RETURN for next location)
   2134 94
   2135 7F
   2136 FF
   2137 7F
   2138 FF
   2139 FF
   213A 08
   213B 19
   213C 19
   213D 32
   213E 31 .	(period terminates the "S"et command)
   -G0

   A>

The modified image can be saved immediately after exiting DDT:

   A>SAVE 38 CPMNEW.COM

The new image can be written to disk using SYSGEN

   A>SYSGEN CPMNEW.COM