Radio Shack TRS-80 Service Manual page 33

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TRS·BO
®
Service Manual
Memory Map Decoding
External subsections of the Data Terminal are memory mapped.
Each subsection is mapped to a range of addresses that is
directly addressable by the microprocessor. Enabling of
each subsection is preformed by map decoder U-16. This chip
has four of the higher order address bits connected to it.
Depending of the state of the inputs, one of the outputs of
U-16 will go LOW, selecting a subsection.
The subsections are listed below, along with the bit
patterns necessary to select them. Notice bit A-13 must be
HIGH before any of the subsections are enabled. In general,
the decoder segments the subsections into 1 K byte "pages."
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A-13 A-12 A-11 A-1/J Contents
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Video RAM
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Low video memory select
High video memory select
Non-volital array select
Keyboard read select
Keyboard/RS-232 input read
Non-volital RAM write select
Decoded position not used
CRT controller enable.
Table 4
The Video RAM holds the character data for the display. The
CRTC does not read this RAM to format video and generate the
sync signals. The microprocessor will read and write data
to the video RAM, and will pass character data to the CRTC
upon request. The Video RAM is just a holding area the
microprocessor uses to store characters for the CRTC.
The RAM consist of four l/J24 by 4 bit static memories,
structured and mapped in the address decoder map as 2/J48
bytes of eight bit words. Only the microprocessor has
access to this memory.
Non-Volital RAM
A Non-volital RAM gives the DT-1 the ability to retain Set
Up parameters. This non-volital RAM is wired into the
display RAM circuitry just as if it were system RAM. The
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