titus-larsen-titus-1981-apple-interfacing

will now explore the actions that each of these software commands
causes to take place.
The PEEK and POKE instructions operate in a very similar manner.
In each, an address is specified, requiring 16 bits of information.
During the execution of either instruction, the address information
contained within the command is transferred to external devices
on the address-bus lines, Al5-AO. In this way, the 1/0 device address
is available to all of the devices and circuits that are connected to
these address lines, both memory and I/ 0 devices.
When a POKE instruction is used in a program, the data value is
also output by the 6502 chip, but on the data-bus lines, D7-DO. Once
the data bits and the address bits are "stable" or present on their respective
buses in useable form, the 6502 asserts the READ/WRITE
signal on the control bus. This synchronizes the acquisition of the
data by the I/0 device that was addressed. Of course, external circuitry
is required to "capture" the data, as well as to identify the selected
I/ 0 device and synchronize it with the 6502-based system. A
timing diagram for these signals, as they appear on the 6502 system,
in this case the Apple, is shown in Fig. 1-4. Of course, the POKE
command involves many assembly-language instructions, and the
timing diagram shows what happens only during the time of the
actual data transfer. At this point, we are only concerned with what
the 6502 does during a POKE operation.
When a PEEK instruction is executed, the data is not contained in
the instruction, but is acquired from an external I/0 device. Only
the address is specified. The 16-bit address is placed on the address-
i---Tcvc ----i
01
(APPLE 01)
02
(APPLE 00)
DATA
FROM MPU
Fig. 1-4. Write operation signal relationships. (See Appendix C.)
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