Texas Instruments 99/4A - Page 46

ROM & RAM MEMORY
GENERAL DESCRIPTION
The 99/4A Home Computer uses 8K bytes of read only memory for
execution of fast system routines. It also has 256 bytes of
random access memory which is used as a scratch pad memory.
READ ONLY MEMORY
The 99/4A uses ths TMS 4732 as a read-only memory. This device
contains 32K bits of read-only memory. It is housed io a 24 pin
dual in line package and uses a single 5 volt supply voltage. 12
address lines are used to select 4K bytes of 8 bits. The device
uses two chip select inputs, which both have to be active (low) to
select the device.
READ-AND-WRITE MEMORY
The 99/4A uses the MCM 6810 as random access memory. The MCM 6810
is housed on a 24 pin dual in line package and uses a single 5
volt supply voltage. 7 address lines are used to select 128
fields of 8 bits. 6 pins are available for device selection, of
which two have to be low and four have to be high to select the
RAM. Another pin (R/W) is provided to distinguish between writing
and reading of data.
HARDWARE DESCRIPTION
Hardware connections to ROM and RAM memory are given in figure
12,p.38.
RAM MEMORY
Two RAMS (0608 & U609) are connected in parallel to generate a 16
bit data bus, as is required by the TMS 9900. The devices are
selected by the RAMCS signal, generated by the memory selection
logic. Distinction between reading and writing is made by
connecting the WE signal of the TMS 9900 to the R/W PIN 16.
Address lines AS through A14 are used to select the proper memory
1 ocations .
ROM MEMORY
Both ROMS are connected in parallel to generate the required 16
bit data bus. The ROMS are selected by the ROMEN signal,
generated by the memory selection logic. This signal is applied
on pin 20. Address lines A3 through A14 are used to select the
proper memory locations.