Spectrum ROM: Routine at 1601

1601: THE 'CHAN-OPEN' SUBROUTINE

Used by the routines at SAVE_ETC, SA_CONTRL, PO_SCR, CLS, CL_ALL, ED_EDIT, ED_UP, MAIN_EXEC, LIST, LPRINT, STR_ALTER, INPUT, str and read_in.

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.

Input
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

160C

JR NZ,CHAN_OP_1

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.