80-Bus News

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EXAMPLE 2 MACHINE CODE SUBROUTINE TO GET COORDINATES

Subroutine to request and get the coordinates of the light pen from the IVC. This routine is for Microsoft’s MBASIC

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MAKINT EQU 105H s;Address of MAKINT routine Ivest QU OB2H ;IVC status register

IvepDt EQU OBIH sIVC data register

ESCAPE EQU 01 BH sAscii “ESC” code

This routine is invoked by the USR command and returns the coordinates as an integer of the form:--- ROW*256 + COLUMN

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LPEN: IN A, (IVcstT) 3;See if IVC ready to rec. RRCA ;Plag to carry JR C,LPEN sLoop if not LD A, ESCAPE ;Send “ESC” OUT (IVCDT),A LPENO: IN A, (Ivest) ;Wait again RRCA JR C, LPENO LD A,"P” ;Now send the “P” OUT (IVCDT),A 3 Command sent, now wait for coordinates LPEN1: IN A, (Ivcst) ;Wait for ROW RLCA JR C,LPEN1 IN A, (IvcDT) s;Read ROW LD H,A ;Put in H LPEN2: IN A, (1Ivcest) sWait for COL RLCA JR C, LPEN2 IN A, (IveDT) sRead COL LD L,A ;Put in L PUSH HL sSave the value LD HL, (MAKINT) sGet address of return routine EX (SP),HL ;Reset HL (MAKINT on stack) RET ‘ sReturn the value END

EXAMPLE 3 – MBASIC

This Basic program, which is well sprinkled with REM statements, is provided as a guide that others can follow. As it is purely for demonstration purposes the program has been coded for clarity.

Note: In line 1020 ADDR holds the address of where the USR routine is stored. Nascom Basic/Nas-Sys users should pick an area within the workspace RAM (0C80H-OFFFH). GM809 users on a Nascom can use the workspace RAM as well, though in their case it is located at OFCOOH-OFFFFH. (NB Your CP/M system size should be 63K or less if you intend to use the workspace RAM).

Por Multiboard users the problem is slightly more complex. Either a CP/M system can be used that is 1K less than the available memory size, or the /M option can be used when loading MBASIC to leave an area of RAM free for the machine code program.