Texas Instruments 99/4A - Page 90

GENERAL DESCRI FT I ON
The 9'-'/4A I/O bus provides maximum flexibility for operation with
external devices. It provides peripherals with memory and CRU I/O
buses. The memory bus (with 8 bit data bus) is used for instruc 
tion -Fetch from control ROM in the peripherals and for data trans 
fer to/from memory mapped peripherals. The CRU bus is used for
peripherals enab 1 e/disab 1 e and for device control and data trans 
fer to/from CRU mapped peripherals.
The TMS 9900 accesses each peripheral to obtain instructions from
the device service routine (DSR) read only memory. Since each
peripheral contains its own DSR, the 99/4A does not have to be de 
signed to anticipate future peripheral requirements . The dual I/O
bus capability, along with interrupt handling and external DSR ? s
provide flexibility at low cost.
MEMORY ALLOCATION
The third memory block (addresses 4000-5FFF) of the 99/4A is pre-
decoded and made available at the I/O port for the peripheral.
Addresses A000 - FFFF are available for memory expansion. For the
speech module (addresses 90XX - 94XX), a predecoded line is avail 
able at the I/O bus.
CRU ALLOCATION
Of the available 4K of CRU bits, the first IK (addresses 0000 -
07FE) are used internally in the Home Computer. The second IK
(addresses 0800 - OFFE) are reserved for future use. The last 2K
addresses 1000 - 1FFE) are reserved for the peripherals to be
plugged in the 1/0 port. A block of 128 CRU bits is assigned to
each peripheral as listed below.
TABLE 18 CRU ALLOCATION
CRU ADDRESS
PERIPHERAL
1 000
Reserved
1 1 00
Disk System
1200
Modem
1 300
Primary RS232
1 400
Not Assigned
1 500
Secondary RS232
1 600
Not Assigned
1 700
HEX BUS (TM)
1 800
Thermal Printer
1 900
Not Assigned
1 A00
Not Assigned
1 BOO
Not Assigned
1 COO
Video Controller Card
1 DOO
IEEE 488 Controller Card
1 E00
Not Assigned
1 FOO
P — Code
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