By Rodnay Zaks

ISBN-10: 0895880156

ISBN-13: 9780895880154

Publication by means of Rodnay Zaks

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BIT CRA 29 6502 APPLICATIONS BOOK BMI BVC IRQA7 NOT Al IRQA6 A2 IRQ FOUND (Bit 6) IRQA7 Al IRQ FOUND (BIT 7) NOTAl BIT BMI BVC CRB IRQB7 NEXT2 SAME FOR PORT B IRQB6 B2 IRQ FOUND (BIT 6) IRQB7 Bl IRQ FOUND (BIT 7) NEXT2 NEXT 6520 BIT Fig. 2-13: Identifying the Ports The "BIT" instruction is used to test whether either bits 6 or 7 are a "I". '' The BIT instruction sets V flag and the N flag, so that these two flags can now be tested; BMI BVC IRQA7 NOTAl BIT7 = I NO INTERRUPT FOUND If none of the flags were set, a branch will occur to NOT Al, where the CRB will be checked.

The Interval Timer The interval timer is equipped with an 8-bit register, and may be used in one of four modes. Depending on the values AO and Al of the A2 Al AO 0 0 0 0 0 0 DORA 0 0 f 0 BUFFER A Note: A3 specifies whether interrupt is used. BUFFER B DDRB 0 0 TIMER 1T (W)TIMERBT (R)INTFLAG 0 TIMER64T TIMER 1024T (R)INTFLAG +IRQto PB7 NOIRQtoPB7 +IRQto PB7 NOIRQtoOB7 Fig. 2-37: 6530 Memory Map 61 6502 APPLICATIONS BOOK address lines, it will count in increments of 1, 8, 64, 1024 times the system clock.

2-26: Reading Data When Ready A Simple Input Example Let us specify a low-to-high ''ready'' transition from the peripheral, and an input configuration on Port A (see Fig 2-26). Whenever the data is ready, it will be read into the accumulator. The program is: LOA STA #0 DORA SET INPUTS 39 6502 APPLICATIONS BOOK WAIT LOA STA #1 PCR LDA AND IFR #$02 BEQ LOA WAIT ORA CAI INTERRUPT LOW-TOHIGH READ INT FLAGS 00000010 MASK BIT 1 FOR CAI READY? READ DATA IN Improvement: Can you modify the two instructions "LDA /FR improve efficiency?

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6502 Applications Book by Rodnay Zaks

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