Now that we have successfully managed to read from a DASD device we can build a general purpose subroutine to perform I\/O. \u00a0We will create a control block to communicate with the I\/O routine. \u00a0The control block will be called I\/O Request (IOR).<\/p>\n
IOR DSECT ,\r\nIORCCW DS A ADDRESS OF CCW\r\nIORUNIT DS XL2 DEVICE ADDRESS\r\nIORSIOCC DS X SIO CONDITION CODE\r\n DS X\r\nIORCSW DS XL8 CHANNEL STATUS WORD\r\nIORSENS0 DS X SENSE BYTE 0\r\nIORSENS1 DS X SENSE BYTE 1\r\n*\r\nIORLEN EQU *-IOR<\/pre>\nThe first two fields are input to the I\/O routine and the remainder are output. \u00a0The IORCC field points to the CCW chain to execute. \u00a0The IORUNIT field contains the device address. \u00a0IORSIOCC returns the\u00a0condition\u00a0code returned form the SIO instruction. \u00a0IORCSW contains a copy of the CSW after the I\/O request is complete. \u00a0If the I\/O operation resulted in a Unit Check condition the I\/O subroutine will perform a sense command and save the first two bytes into IORSENS0 and IORSENS1.<\/p>\n
The I\/O subroutine returns a completion code in Register 15 to indicate the completion status of the I\/O request.<\/p>\n
**********************************************************************\r\n* XIO - PERFORM I\/O SUBROUTINE\r\n* R1 = A(IOR) I\/O REQUEST BLOCK\r\n* ON EXIT R15 CONTAINS A RETURN CODE\r\n**********************************************************************\r\n*\r\nXIO DS 0H\r\n STM R14,R12,12(R13) SAVE CALLER'S REGISTERS\r\n LA R14,XIOSAVE POINT TO OUR SAVE ADDRESS\r\n ST R14,8(,R13)\r\n ST R13,4(,R14)\r\n LR R13,R14\r\n*\r\n LR R10,R1 IOR BLOCK ADDRESS\r\n USING IOR,R10\r\n*<\/pre>\nWe begin by saving the callers registers and then establishing a new save area. \u00a0The IOR address is copied into Register 10 and mapped against the IOR DSECT.<\/p>\n
XC IORSIOCC,IORSIOCC\r\n XC IORSENS0(2),IORSENS0\r\n XC IORCSW,IORCSW<\/pre>\nFirst the output fields in the IOR are cleared.<\/p>\n
L R1,IORCCW POINT TO CCW CHAIN\r\n ST R1,72 SAVE INTO CAW\r\n SLR R2,R2\r\n ICM R2,B'0011',IORUNIT DEVICE ADDRESS\r\n SIO 0(R2) START I\/O\r\n BC 4,SIOCC4 BRANCH IF SIO NOT ACCEPTED\r\n BC 2,SIOCC2 BRANCH IF SIO NOT ACCEPTED\r\n BC 1,SIOCC1 BRANCH IF SIO NOT ACCEPTED<\/pre>\nThe CCW address is saved into the CAW and a SIO is issued against the device. \u00a0If the SIO did not complete with CC=8 a SIO CC code is stored in the IOR and the return code is set to 8.<\/p>\n
WAITIO TIO 0(R2) WAIT FOR I\/O COMPLETION\r\n BC 1,TIOCC1 BRANCH IF ERROR\r\n BC 7,WAITIO KEEP WAITING...\r\n*\r\n MVC IORCSW,64 SAVE CSW\r\n CLC =X'0C00',ORCSW+4 CHANNEL END\/DEVICE END\r\n BE WAITIO20 YES - GOOD COMPLETION\r\n TM IORCSW+4,X'02' UNIT CHECK?\r\n BNO WAITIO15 NO - DON'T NEED TO SO SENSE<\/pre>\nA TIO loop is executed to wait for the I\/O request to complete. \u00a0Once the I\/O has completed the CSW copied into the IOR. \u00a0If any status bits except Channel End and Device End are set it is considered to be an exceptional condition.<\/p>\n
If Unit Check was set we fall through to issue a sense command.<\/p>\n
LA R1,CCWSENSE\r\n ST R1,72 SAVE INTO CAW\r\n SIO 0(R2) START I\/O\r\n BC 7,WAITIO99 ERROR\r\n*\r\nWAITIO10 TIO 0(R2) WAIT FOR I\/O COMPLETION\r\n BC 1,WAITIO99 BRANCH IF ERROR\r\n BC 7,WAITIO10 KEEP WAITING...\r\n*\r\n MVC IORSENS0(2),SENSE SAVE SENSE BYTES 0 & 1\r\nWAITIO15 DS 0H WAIT FOR I\/O COMPLETION\r\n LA R15,4 SET RC=4\r\n B XIOXT<\/pre>\nThe address of the SENSE CCW is stored into the CAW and a SIO is issued to the device. \u00a0A TIO loop is used to wait for the Sense I\/O to complete. \u00a0The first two bytes of the sense data is stored into the IOR. \u00a0We can now exit with a return code of 4.<\/p>\n
SIOCC1 DS 0H\r\n MVI IORSIOCC,X'01'\r\n LA R15,8\r\n B XIOXT\r\n*\r\n*\r\nSIOCC2 DS 0H\r\n MVI IORSIOCC,X'02'\r\n LA R15,8\r\n B XIOXT\r\n*\r\n*\r\nSIOCC4 DS 0H\r\n MVI IORSIOCC,X'04'\r\n LA R15,8\r\n B XIOXT\r\n*\r\n*\r\nWAITIO99 DS 0H\r\nTIOCC1 DS 0H\r\n LA R15,16\r\n B XIOXT\r\n*\r\n*\r\nXIOXT DS 0H\r\n L R1,4(,R13)\r\n ST R15,16(,R1)\r\n*\r\n @PRINT '-------- IOR --------'\r\n @DUMP IORB,IORLEN\r\n @PRINT ' '\r\n @PRINT ' '\r\n*\r\n L R13,4(,R13)\r\n LM R14,R12,12(R13) RESTORE CALLER'S REGS\r\n BR R14 RETURN TO CALLER<\/pre>\nWe finally end up at XIOXT with a return code set in Register 15. \u00a0Here I save the RC into the callers save area so it will be restored into Register 15 before returning to the caller. \u00a0For debugging purposes I have included a dump of the IOR Block. (This is why saving the value in Register 15 was necessary)<\/p>\n
[Next – Searching The VTOC]<\/a><\/p>\n","protected":false},"excerpt":{"rendered":"
Now that we have successfully managed to read from a DASD device we can build a general purpose subroutine to perform I\/O. \u00a0We will create a control block to communicate with the I\/O routine. \u00a0The control block will be called … Continue reading