By Lance A Leventhal

Guideline textual content for meeting language geared in the direction of the Motorola 6809 microprocessor that's present in the Tandy TRS-80 desktop.

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Extra resources for 6809 Assembly Language Programming

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50 Atari Assembly Language Programmer's Guide The compare instructions subtract the contents of memory from the register but does not save the result. The indications of the result are the conditions of the three status flags: N (negative), Z (zero), and C (carry). The condition of these flags indicate whether the register contents are less th an , equal to, or grea ter than the contents of memory location . A very natural follow-up to a compare instruction is to make a decision on the basis of the comparison.

Occasionally we will refer to "whole page boundaries" or "IK and 2K boundaries". A memory location is said to lie on a one page boundary if its address is evenly divisible by 256. A memory address lies on a I K or 2K boundary if its address is divisible by 1024(1 K) or 2048(2K) respectively. In the Atari computers, certain pages of memory are set aside for use by the operating system (OS) and BASIC, while others are used primarily for hardware registers. Which page is chosen for a particular use depends in part on the design of the 6502 microprocessor and in part on choices made by the Atari's designers.

The command does not alter either the accumulator or memory, but records information in the status register as follows: N flag is the original value of bit 7 of the memory byte. V flag is the original value of bit 6 of the memory byte. Z flag is set if the AND operation generates a zero . BIT is a special purpose instruction that is used in communication between the PIA and the 6502. It is used mainly in programs that are written as part of hardware interfacing. Addressing Modes Each of the 151 instructions has its own numeric operation code (op-code, for short).

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6809 Assembly Language Programming by Lance A Leventhal


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