Jameco-electronics Rabbit 3000 Uživatelský manuál stáhnout pdf (Strana 48)

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User’s Manual 39

3.3.8 Input/Output Instructions

The Rabbit uses an entirely different scheme for accessing input/output devices. Any

memory access instruction may be prefixed by one of two prefixes, one for internal I/O

space and one for external I/O space. When so prefixed, the memory instruction is turned

into an I/O instruction that accesses that I/O space at the I/O address specified by the 16-

bit memory address used. For example

IOI LD A,(0x85) ; loads A register with contents

; of internal I/O register at location 0x85.

LD IY,0x4000

IOE LD HL,(IY+5) ; get word from external I/O location 0x4005

By using the prefix approach, all the 16-bit memory access instructions are available for

reading and writing I/O locations. The memory mapping is bypassed when I/O operations

are executed.

I/O writes to the internal I/O registers require only two clocks, rather than the minimum of

three clocks required for writes to memory or external I/O devices.

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