MORE ON THE SCB					
								
		Gordon Wilk
		6707 Springpark Ave
		Los Angeles CA 90056
		213	645-6419


A number of execellent articles on the System  Control Block of 
CPM Plus have been published in FOGHORN, yet there is more to
tell.

The DRI Programmer's Guide insists that we use BDOS function 49
to get or set values in the SCB, yet the CCP does not do it this
way.  It turns out that a particular location in the SCB (offset
of CDh) contains the address of the SCB. We can get this address 
with function 49 once and then use it to read and write the SCB 
directly.  This location contains the address but it is easier to 
just use the high byte and refer to all locations by their low 
byte address on this page.  Any SCB value can be easily located by 
placing the high byte in H and the offset in L.  The CCP does it 
this way.

To find the SCB base:

	LXI	D,SCBPB
	MVI	C,49
	CALL	05
	SHLD	SCBBase
	RET
SCPBP	DB	03Ah,00	

There is no easy way to make a DOS call in BASIC so use this code
once or you can assume that SCBBS% = &HF800 (probably).

Now we have the base page address stored in SCBBase.  To access
any value in the SCB

	LHLD	SCBBase		;Get the page address
	MVI	L,offset	;Move an offset into L
				;We are now pointing to the value
	MOV	A,M		;Load it into register A
	RET
in BASIC:
	scbname% = PEEK(SCBBS% + offset)
  e.g.  VERSION% = PEEK(SCBBS% + &HA0)

To set any value in the SCB

	LHLD	SCBBase
	MVI	L,Offset
	MOV	M,A
	RET
in BASIC:
	POKE SCBPG% + offset, value
  e.g.  LISTFLG% = &HD4
	POKE SCBPG% + LISTFLG%, 1	


This is the way the CCP does it; the BDOS have direct access
since BDOS was told the base address by GENCPM when the system 
was generated.

In the list below are the values in the SCB with their addresses
expressed as offsets from the base page. On my system this is
F800h, it probably is on most Osborne Execs -- you can address
the SCB directly without this procedure, but using the procedure
allows your code to run on any CPM Plus system.

A number of the values in the SCB are listed only as "reserved
for system use" in the documentation.  I have attempted to
describe these based on their use by the CCP and BDOS using the
dissassembled code produced by Clark Calkins' SCG31 (see end of
article for acknowledgement and more information).  Some of them
may have multiple uses and I have only listed those I am sure of.
Most variables documented by DRI, I have left without comments. A 
few, I have not figured out and would like to hear from anyone 
who has.

The offsets listed below are relative to the SCB base page       
address. Official SCB addresses are 9Chex smaller.

OFFSET	NAME	     LENGTH	DESCRIPTION
------  ----	     ------     -----------	

9C-A0   HASHBYTES	These are used by the directory and
			filename routines.
9C	HashChek	1	Hash value for directory check
9D	HashDrve	1	Drive being checked
9E	HashName	2	File being checked
A0	HashExt		1	Extant being checked

A1	Version		1	CCP resets to 31h at warm boot 
A2	Users		4
A6	???		5	May be used by SUBMIT.COM?
AB	SavDrv		1	Save current Drv here if SUBMIT 
				may change it.
AC	RetCode		2	CCP resets to 0 if not chaining

   The remainder of a multiple command line (the part after the !) 
   is stored as an RSX. RSXPAGE and NXTCMD are used to locate
   this special area.
AE	RSXPage		1	HiByte of RSX address

AF	DefDrv		1	Default drive saved here
B0	DefUser		1	Default user # saved here

B1	NxtCmd		2	Address of rest of multiple input
				line

B3-B4	CCPFLGS		Used by the CCP for a number of  purposes.
			Involved in SUB file, multiple cmd line
			processing and chaining.  Note that Flag2
			overlagps SavBuff.
B3	Flag1		1	Bit 7: chain requested
				Bit 6: use default User & Drv
				Bit 1: remove RSXs
B4	Flag2		1	Bit 7: cmd line not finished
				Bit 6: Use current User code
				Bit 5: reset dieks
				Bit 4&3: COM/PRL/SUB code
				Bit 0: SUB file active

B4	SavBuff		2	Address of previously saved input line
				used to recall last command.

B5	Profile		1	Overlaps. If Bit 1 is set, which
 				is only on a cold boot, the
 				PROFILE.SUB command is executed.
B6	ConWidth	1
B7	CurCol		1
B8	PageLength	1
B9	???		1	Default Page length???
BA	BuffPtr		2	input line pointer
BC	KBPtr		2	keyboard input line pointer
BE	@CIVEC		2
C0	@COVEC		2
C2	@AIVEC		2
C4	@AOVEC		2
C6	@LOVEC		2
C8	PageMode	1
C9	DefPgMode	1	Page mode stored here by CCP 
CA	CTRL$H		1
CB	RUBOUT		1
CC	KeyStat		1	Keyboard status for submit mode input
CD	???		2
CF	ConMode		1	Console mode CCP resets to 1 @ WarmBoot
D0	RSXInput	1	Flag set if an RSX is providing
				console input. Used by SUBMIT and
				GET. CCP resets to 0 @ warm boot

D1	@BNKBF		2	Address of bank buffer
D3	Delim		1	CCP resets to '$' on warm boot
D4	OutFlag		1
D5	KeyBdFlg	1	Keyboard locked flag
D6	SCBBase		2	The base of this table
D8	@CRDMA		2
DA	@CRDSK		1	CCP resets to DefDrv at warmboot
DB	@VINFO		2	(DE) on entry to this function ->fcb
DD	@RESEL		1	
DE	@MEDCHG		1
DF	@FX		1	Register C on entry to BDOS
E0	@USRCD		1	CCP resets to DefUser at Warm boot

E1-E5   SEARCH		Used by BDOS functions 17 & 18
E1	Entry		2	directory entry location for searching
E3	SrchFCB		2	fcb adr for searches
E5	MtchCnt		1	char count to match

E6	@MLTIO		1	CCP resets to 1 on warm boot
E7	@ERMDE		1	CCP resets to 0 on warm boot
E8	DrvChain	4
EC	TempDrv		1
ED	@ERDSK		1
EE	??		2
F0	@MEDIA		1	door open flag
F1	??		2
F3	@BFLAGS		1	Bit 7: error message length
F4	@DATE		2
F6	@HOUR		1
F7	@MINUTE		1
F8	@SECONDS	1
F9	Common		2	address of common memory base
FB	?ERJMP		3
FE	@MXTPA		2

Perhaps the most interesting unpublished locations are "DefDrv"
and "DefUser".  Applications programs either directly or via
BDOS calls may set and reset the current drive and user number 
at will.  The CCP will always reset them to the default when
the application program ends.  The CCP does this by  setting
@USERCD and @CRDSK to the values in AFh and B0h on warm boot.
When the CCP changes the user (the USER command) or the current
drive ("B:"), it also sets "DefUser" or "DefDrv". In the normal
course of events only the CCP can permanently change either the
User or default drive.  A sneaky  application program can do
the same thing by changing "DefUser" and/or "DefDrv".  The
current user and drive settings will not be affected, but the 
CCP will reset them on warmboot.


This work was based on the disassembled code produced by SCG31, a 
program copyright by C.C.Software, 1907 Alvarado Ave Walnut Creek
CA 94596, 415 939-8153.  It costs $75 and is worth it for anyone
knowledgeable in assembly language and curious about the way the
operating system works.

Labels beginning with "@" or "?" are copyright by DRI.
 
 language and curious about the way the
operating system wo