Texas Instruments 99/4A - Page 23

CENTRAL PROCESSING UNIT
GENERAL DESCRIPTION
THE 99/4A Home Computer use the Texas Instruments TMS 9900
mi preprocessor . It provides the system with a 15 bit address and
18 bit data bus for communication with external memory. It also
has input and output pins for serial in and output, interrupt
handling and memory control (see -Figures 3 and 4, p. 20-23).
MEMORY INTERFACING
The TMS 9900 inter-Faces with memory by means of a 16 bit data bus
(D0-D15) , the 15 bit address bus (A0-A14) and the -Following
control signals.
MEMEN MEMORY ENABLE. When active (low), MEMEN indicates that the
address bus contains a memory address
WE
WRITE ENABLE,
write data is
memory .
When active (low)
WE indicates that memor
available from the TMS 9900 to be written into
DBIN DATA BUS IN. When active (high), READY indicates that
memory will be ready to. read or write d^uring the next clock
cycle. When not ready is indicated during a memory
operation, the TMS 9900 suspends further operation (e.g.
enters a wait state) until READY becomes active again, after
which the memory read/write cycle is completed. This signal
enables the use of slow memory devices with the TMS 9900.
WAIT WAIT. When active (high), WAIT indicates that the TMS 9900
has entered
from memory.
a wait
state because of a not-ready condition
Timing diagrams for read and write cycles, with and without wait
states, are given in Figures 5-8, p. 25-28.
DIRECT MEMORY ACCESS
Performing memory access without interference of the TMS 9900 is
called direct memory access (DMA). For this purpose the following
control lines are available.
HOLD When active (low), HOLD indicates to the processor that an
external controller desires to use address, data and memory
control signals. The TMS 9900 enters the hold state
following a hold signal when it has completed its.present
memory cycle. The processor then places the address and
data buses in the high-impedance state along with MEMEN, WE
and DBIN and responds with a hol'd ackowledge signal (HOLDA) .
When HOLD is removed, the processor returns to normal
operat ion .
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