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This subroutine is called with the A register holding a valid stream number - normally +FD to +03. Then depending on the stream data a particular channel will be made the current channel.
A Stream number
CHAN_OPEN 1601 ADD A,A The value in the A register is doubled and then increased by +16.
1602 ADD A,$16
1604 LD L,A The result is moved to L.
1605 LD H,$5C The address 5C16 is the base address for stream +00.
1607 LD E,(HL) Fetch the first two bytes of the required stream's data.
1608 INC HL
1609 LD D,(HL)
160A LD A,D Give an error if both bytes are zero; otherwise jump forward.
160B OR E
This entry point is used by the routine at STR_ALTER.
Report O - Invalid stream.
REPORT_O 160E RST $08 Call the error handling routine.
160F DEFB $17
Using the stream data now find the base address of the channel information associated with that stream.
CHAN_OP_1 1610 DEC DE Reduce the stream data.
1611 LD HL,($5C4F) The base address of the whole channel information area (CHANS).
1614 ADD HL,DE Form the required address in this area.
This routine continues into CHAN_FLAG.
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