The BBC and Master Computer Public Domain Library

97B1 PLOT

Submitted by Steve Fewell

Description:

Get the Integer result of the expression after the PLOT keyword (at BASIC Text pointer A location).
If a string value was found then issue a 'Type mismatch' error.

Push the LSB of the Integer value (&2A) to the 6502 stack. This is the first PLOT paramater (i.e. the PLOT type).
Note: Only the 1-byte LSB value of the Integer is used - the rest of the Integer value (if supplied) is ignored.
Call routine &96AC to check for a comma ',' (issuing an error if not found), and get the Integer result of the
expression after the comma (issuing 'Type mismatch' error if a String value was found).

[&97BA] Push the second Integer PLOT parameter value (in the IWA bytes &2A-&2D) to the BASIC stack.
Call routine &96AC to check for a comma ',' (issuing an error if not found), and get the Integer result of the
expression after the comma (issuing 'Type mismatch' error if a String value was found). The IWA now contains the third
PLOT parameter.

Call &9B96 to check for the end of statement (at BASIC Text Pointer B). If the next character after the third PLOT
parameter value is not ':', '<cr>' or 'ELSE-token' then issue 'Syntax error', as the statement was not terminated
correctly.

Send ASCII character #&19 to the OS Write character routine (to start a VDU 25 command).
Retrieve the first PLOT parameter from the 6502 Stack (this is the 1-byte Integer that specifies the PLOT type).
Send the first PLOT parameter (retrieved above) to the OS Write Character routine (the Operating System will take this
value as the first parameter to the VDU 25 command.

Retrieve the second PLOT parameter from the BASIC Stack to locations &37-&3A (this is a 4-byte Integer value).
Send byte 1 (LSB) of this second PLOT parameter (location &37) to the OS Write Character routine (the Operating System
will take this value as the second parameter to the VDU 25 command.
Send byte 2 of this second PLOT parameter (location &38) to the OS Write Character routine (the Operating System
will take this value as the third parameter to the VDU 25 command.
Note: Only the first 16-bits of the 4 byte Integer are used (bytes 3 and 4 are ignored).

Send byte 1 (LSB) of the third PLOT parameter (IWA byte &2A) to the OS Write Character routine (the Operating System
will take this value as the forth parameter to the VDU 25 command.
Send byte 2 of the third PLOT parameter (IWA byte &2B) to the OS Write Character routine (the Operating System
will take this value as the fifth parameter to the VDU 25 command.
Note: Only the first 16-bits of the 4 byte Integer are used (bytes 3 and 4 are ignored).

Lastly, jump to &9005 to start processing the next program statement.

Disassembly for the PLOT routine