Interrupt control block (ICB)

The SAM ICB, used by BAM, contains channel programs and status information when using chained scheduling. Chained scheduling is used only on tape and unit record devices. The ICB is pointed to by the DCBIOBA field of the DCB.

Table 1. SAM Interrupt Control Block (ICB)—Description and Format
Offset Dec(Hex)	Bytes and Bit Pattern	Field Name	Description
0 (X'00')	*	ICB	SAM interrupt control block.
0 (X'00')	1	ICBNFLG1	Flag byte.
	X'80'	ICBPRTOV	PRTOV has occurred.
	X'40'	ICBWRITE	Write operation in process.
	X'20'	ICBREAD	Read operation in process.
	X'10'	ICBUPDAT	Block is to be updated.
	X'08'	ICBBKSPC	ICB being used for backspace, CNTRL, NOTE, or POINT.
	X'04'	ICBSPAN	This record is a spanned record.
	.... ..x.		Reserved.
	X'01'	ICBFIRST	This is first ICB on chain.
1 (X'01')	3	ICBNICBB	Address of next ICB on chain.
4 (X'04')	4	ICBNECB	Event control block.
8 (X'08')	1	ICBFLAG1	Flag byte.
	X'80'	ICBDATCH	Data chaining used in channel program.
	X'40'	ICBCMDCH	Command chaining used in channel program.
	X'20'	ICBERRTN	Error routine is in control.
	X'10'	ICBRPSTN	Device is to be repositioned.
	X'08'	ICBCYCCK	Cyclic redundancy check (tape only).
	X'04'	ICBIOERR	I/O error has occurred.
	X'02'	ICBUNREL	This I/O request is unrelated (nonsequential).
	X'01'	ICBRSTRT	Restart address in ICB to be used.
9 (X'09')	1	ICBFLAG2	Flag byte.
	X'80'	ICBHALT	Halt I/O has been issued by SVC purge routine.
	X'40'	ICBSENSE	Issue sense command after device end occurs.
	X'20'	ICBPURGE	ICB has been purged; allow I/O to quiesce.
	X'02'	ICBSTATO	Device end status has been ORed with channel end status; graphics device.
	X'01'	ICBPNCH	Turned on by QSAM when error recovery is to be provided for the 2540 card punch.
10 (X'0A')	1	ICBSENS0	First sense byte.
11 (X'0B')	1	ICBSENS1	Second sense byte.
12 (X'0C')	1	ICBECBCC	Completion code for this I/O request.
13 (X'0D')	3	ICBECBPB	Address of ECB to be posted on I/O completion.
16 (X'10')	1	ICBFLAG3	Error routine flag byte.
17 (X'11')	7	ICBCSW	Low order seven bytes of CSW at channel end.
24 (X'18')	4	ICBSTART	Address of channel program.
24 (X'18')	1	ICBSIOCC	Bits 2 and 3 = condition code from SIO.
28 (X'1C')	2	ICBINCAM	Value used to increment block count on tape.
28 (X'1C')	1	ICBCRDCC	Optical reader; data check error count.
29 (X'1D')	1	ICBCRILC	Optical reader; incorrect length error.
30 (X'1E')	1	ICBINDIC	Special count indicators.
	X'80'	ICBVOLFL	End of volume with read/write error.
	.xxx xxxx		Reserved.
31 (X'1F')	1		Reserved.