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/***** MULTI8 DECTAPE EMULATOR *********
/LAST EDIT: 14-JULY-75, JV
/LAST EDIT:  4-JULY-75, ELC
/LAST EDIT: 30-JUNE-75, ELC
/LAST EDIT: 25-JUNE-75, JV+ELC
/LAST EDIT:  1-JUNE-75, JV
/LAST EDIT:  8-JANUARI-75, ELC
/LAST EDIT: 12 OKTOBER 74, ELC
/DECTAPE EMULATOR FOR BG

/NAME:	TE ("T^100+"E&3777)
/LENGTH:ONE PAGE
/FUNCTION:THIS MODULE DOES THE DECTAPE TRANSFERS FOR
/	THE OS8 BACKGROUND. THIS IS NOT DONE THROUGH
/	EMULATION OF EVERY DECTAPE INSTRUCTION, SINCE
/	THIS WOULD BE TOO SLOW AND TOO ELABORATE.
/	THE FOLLOWING SIMPLE SOLUTION IS TAKEN:
/	THERE IS A SPECIAL DECTAPE HANDLER FOR OS8
/	WHICH REPLACES THE CONVENTIONAL ONE. THIS 
/	HANDLER PICKS UP THE ARGUMENTS FOLLOWING THE
/	CALL AND COMPUTES THE DECTAPE UNIT NUMBER.
/	IT THEN ISSUES A GIANT IOT (6770) WHICH IS
/	RECOGNISED BY THE FOREGROUND.
/CONCEPT:THE MODULE IS ENTERED VIA THE SPECIAL FUNCTIONS IOT.
/	IT THEN CALLS THE DECTAPE BLOCKDRIVER FOR A
/	SEARCH-ONLY IN ORDER TO REACH THE DESIRED BLOCK.
/	IN THIS TIME THE CURRENT BG IS INACTIVE AND
/	EVEN SWAPPED BACK TO THE DISK.
/	WHEN THE RIGHT BLOCK COMES UP, THE BG IS REQUESTED
/	IN CORE AGAIN, AND THE TRANSFER COMPLETED
/	IN A SINGLE RUSH. IN THIS TIME THE BG CANNOT BE
/	SWAPPED. (EMULATE)
/ERRORS:ERRORS ARE DEFERRED TO THE BACKGROUND BY TAKING THE
/	HANDLER ERROR-RETURN.
/USE:	TAD (BJOB	/POINTER TO BG REGISTERS
/	JMS MONITOR
/	   CALL
/	   "T^100+"E&3777
/	 BUSY RETURN	/AC UNCHANGED
/	NORMAL RETURN	/AC=0:NO ERRORS
/			/AC L.T. 0: ERRORS;

/THE BG ISSUES THE 'GIANT IOT' 6770
/FOLLOWED BY THE DATATRANSFER VECTOR:

/TAD (DTAFUN
/6770
/RLL.LLL.FFF.###	/READ/WRITE;LENGTH(PAGES);FIELD;UNIT
/BUFFER ADDRESS IN CORE
/BLOCK# ON DECTAPE
/HERE IS WHERE THE ERROR RETURN GOES TO
/HERE GOES THE NORMAL RETURN TO
*200
DTUAC,	"T^100+"E&3777	/TE
DTUPC,	200		/ONE PAGE;NO CONNECTS
DTIDTV,	DTDTV		/RELOCATED POINTER
DTUSTAT,0		/PNTR TO STATUS REG. OF BG

/NOTE THAT DF POINTS TO MONFLD MOST OF THE TIME
TE,	DCA DTUSTAT
	TAD DTUSTAT	/SET UP SOME POINTERS
	TAD C3
	DCA DTUPC
	AC0002
	TAD DTUPC
	DCA DTUAC
/FETCH THE POINTER TO DTV
	TAD DTUSTAT
	TAD (UDTV-1
	DCA AUTO10
	TAD I AUTO10
	AND C7770
	DCA DTFUN
	TAD I DTUAC
	AND C7		/GET UNIT NUMBER
	TAD DTFUN
	DCA DTFUN
	TAD I AUTO10
	DCA DTDTV+1	//
	TAD I AUTO10
	DCA DTBLK
IFNZRO BGMAX-1 <
/LOOK INTO THE TAPETABLE AND SEE HOW FAR WE HAVE TO GO. IF IT'S
/MORE THAN 50 BLOCKS, MAKE THE BG INACTIVE AND START A SEARCH-ONLY
/REQUEST UNTIL WE ARE QUITE NEARBY. IF IT'S ONLY A SHORT DISTANCE
/DO THE TRANSFER RIGHT AWAY, KEEPING THE BG IN EMULATE STATUS.
/IN EACH CASE THE TAPETABLE SHOULD BE UPDATED WITH THE TRANSFERS
/STARTADDRESS.

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