CP/M 3 System Control Block
Jim Lopushinsky, June 24, 1986
HTMLized by John Elliott, 3 November 1995
[John's comments in square brackets]
Here is a complete description of the fields in the CP/M 3.1
System Control Block, as far as I can determine. The
undocumented fields were discovered while dissambling the CCP,
Resident BDOS, and Banked BDOS. Digital Research documents the
SCB as 100 decimal bytes, but in fact it is 152 or 98 hex bytes
long. The SCB occupies the last 152 bytes of the resident
portion of the BDOS (just before the BIOS). It begins at xx68h,
where xx is the last page in the BDOS; therefore, offset 62h-63h
(top of TPA address) is at xxFEh, just before JMP BOOT bios
entry. The address of the SCB can also be determined as follows:
LXI D,SCBPB
MVI C,49
CALL BDOS ;page address of SCB in reg H
...
SCBPB: DB 3AH ;this is where SCB address is
DB 0 ;get operation
To access a field in the SCB, use the above code sequence, then
load register L with the field offset as determined from the
table below. HL then contains the address of the field.
To accomodate RSXs that modify the BIOS character I/O jump
vectors (GET and PUT RSXs), a second jump vector for WBOOT,
CONST, CONIN, CONOUT, and LIST are
included in the SCB. Only the
banked BDOS uses these vectors. Each entry consists of 2 3-byte
entries which are normally jumps. An RSX can change the first
JMP to an LXI H,addr, where addr is the
redirected address of the
particular routine in bank 1. The second JMP always points to a
Resident BDOS bank switch routine which switches to bank 1, calls
the routine as loaded in HL, and on return, switches back to bank
0 and returns to the banked BDOS. The banked BDOS always calls
these 5 BIOS routines via the SCB jump table. This feature can
be used by BYE RSXs for safely redirecting console I/O in RCP/M
systems.
[This feature is obsolete in ZPM3
and the bytes are used for other things]
OFFSET TYPE Description
68H inst. Warm boot jump vector for the banked BDOS.
Normally a jump to the BIOS WBOOT vector, but can
be changed to LXI H,wboot-addr to redirect Warm
boots to a routine in bank 1.
6BH inst. Jump instruction to resident BDOS bank switch
routine for redirected WBOOT.
6EH inst. Console status jump vector for the banked BDOS.
Normally a jump to the BIOS CONST vector, but can
be changed to LXI H,const-addr to redirect console
status to a routine in bank 1.
71H inst. Jump instruction to resident BDOS bank switch
routine for redirected CONST.
74H inst. Console input jump vector for the banked BDOS.
Normally a jump to the BIOS CONIN vector, but can
be changed to LXI H,conin-addr to redirect console
input to a routine in bank 1.
77H inst. Jump instruction to resident BDOS bank switch
routine for redirected CONIN.
7AH inst. Console output jump vector for the banked BDOS.
Normally a jump to the BIOS CONOUT vector, but can
be changed to LXI H,conout-addr to redirect
console output to a routine in bank 1.
7DH inst. Jump instruction to resident BDOS bank switch
routine for redirected CONOUT.
80H inst. List output jump vector for the banked BDOS.
Normally a jump to the BIOS LIST vector, but can
be changed to LXI H,list-addr to redirect list
output to a routine in bank 1.
[82H ZPM3 User number to load file, used by ZCCP loader]
83H inst. Jump instruction to resident BDOS bank switch
routine for redirected LIST.
[83H ZPM3 Address of the Z-System environment, 0 if running
vanilla CP/M]
[85H ZPM3 ZPM3 flags, see ZPM3 documentation]
86H-8FH Unknown.
90H word Bit mapped vector of drives with open files.
92H word Bit mapped vector of drives accessed.
94H-97H Unknown.
98H word BDOS entry point address.
9AH-A0H Used by directory hashing routines. Use Unknown.
A1H byte CP/M version. Contains 31H.
A2H-A5H 4 bytes for user use. CCP+ as written by me uses
offsets A4H and A5H as follows:
A4H byte Bit mapped:
0-4 Submit user number+1. 0 = use current user.
5 reserved.
6 $$$.SUB needs to be erased by the CCP.
7 Directory name display flag. 1 = on.
A5H byte Bit mapped:
0-4 Load function user number+1. 0= current user.
5 reserved.
6 PRL file load
7 Library member load.
A6H-AAH Unknown.
ABH byte Submit file drive.
ACH word Program return code.
AEH byte Base page of RSX containing the 2nd of multiple
commands (next to be executed).
AFH byte CCP drive.
B0H byte CCP user number.
B1H word If non-zero, address of 2nd of multiple commands.
B3H byte Bit mapped CCP flags:
0 Submit flag
1 RSX flag
2-5 Unknown
6 Set CCP drive/user to current drive/user.
7 Chain flag. Next command is taken from
default DMA buffer (80H).
B4H byte Bit mapped CCP flags:
0-1 Display command source drive/user.
2 Unknown
3-4 File type search order:
00 .COM only
01 .COM, then .SUB
10 .SUB, then .COM
11 .PRL, then .COM
5 Reset disk system
6 Reset page mode to default
7 CCP is executing (used by ^W recall)
B5H byte Bit mapped CCP flags:
0 Unknown
1 Cold start flag (0=cold)
2-7 Unknown
B6H byte Console width [eg 79 for 80-column screen]
B7H byte Current console column position
B8H byte Console page length [eg 23 for 24-line screen]
B9H Unknown
BAH word If non-zero, address of redirected console input
characters.
BCH word If non-zero, address of next line of redirected
console input characters.
BEH word Bit mapped console input physical devices.
C0H word Bit mapped console output physical devices.
C2H word Bit mapped auxiliary input physical devices.
C4H word Bit mapped auxiliary output physical devices.
C6H word Bit mapped list output physical devices.
C8H byte Console page mode (0 = page pause).
C9H byte Default page mode.
CAH byte Ctrl-H mode (backspace). [0 => backspace, else echo
deleted character]
CBH byte Rub/Del mode. [0 => Echo deleted character, else
backspace]
CCH-CEH Used by console routines. Use unknown.
CFH word Console mode.
D1H word Address of 128 byte buffer in common memory. This
buffer is used only during BDOS function calls by
the system, so it can be used freely between BDOS
calls as a scratch buffer. Also used by the BIOS
during warm boot.
D3H byte Ouput delimiter.
D4H byte List echo flag (non-zero = echo console output).
D5H byte Scroll flag (used by console routines).
D6H word Address of System Control Block.
D8H word Current DMA address.
DAH byte Current drive.
DBH word Current FCB address (in common memory).
DDH byte FCB error flag.
DEH byte Same drive flag.
DFH byte Current BDOS function number.
E0H byte Current user number.
E1H word Last directory slot number accessed (first = 0)
E3H word Address of FCB for function 18 (search next).
E5H byte Function 17, 18 search type (0 = ? in drive code,
0FH = normal search).
E6H byte Multi-sector count.
E7H byte BDOS error mode.
E8H 4 bytes Drive search chain. [0=Current, 1-16 = drive A-P,
255=end of chain]
ECH byte Temporary file drive. [0=Current, 1-16 = drive A-P]
EDH byte Error drive.
EEH-EFH Unknown.
F0H byte Drive door open flag [Set by the BIOS if a disc
may have been changed since the last access]
F1-F2H Unknown.
F3H byte Bit mapped BDOS flags:
0-5 Unknown.
6 Single allocation vectors.
7 Expanded error messages.
F4H word Date (days since Jan 0, 1978).
F6H byte Hour (BCD).
F7H byte Minute (BCD).
F8H byte Second (BCD).
F9H word Common memory base address (non-banked = 0).
FBH inst. Error message jump instruction to a routine in the
banked BDOS.
FEH word Current top of TPA (points to entry in lowest
RSX).
