A main memory PART2-
Read-only memory 2.4
2.4.1 mask ROM
2.4.2 PROM
One-time programmable fuse burned to 0, continue to 1: burn can not be restored
2.4.3 EPROM
Erasable, rewritable
2.4.5 Flash (Flash Memory Flash Memory)
Cheap, highly integrated, electrically erasable
A memory connected to the CPU 2.5
2.5.1 storage capacity expansion
## monolithic memory chip capacity is limited, a large capacity even up ~
(1) bit extension
Increasing memory word length: chip select lines simultaneously guarantee
Range of data bit lines, a maximum bit number can be transmitted simultaneously, thus increasing the D
(2) Extended Word
Increasing the number of memory words: ensure not work simultaneously, to come up with a 1K are 10 address lines, a total of 11, for selecting a redundant
A10 there is a NAND gate, the word line address belongs, 2K divided into two parts: the address 0 beginning (000,001,010,011) and a beginning address (100,101,110,111)
(3) words, bit extension
A memory connected to the CPU 2.5.2
Connection (1) the address lines
Connection (2) of the data line
CPU and memory chip data lines, not necessarily equal, to the memory chip (small attendant) is expanded position, equate
(3) read / write command line connecting
(4) connected to the chip select line
(5) reasonable choice of memory chips
(6) Other: Timing & Load
[Example of]
## 74138 decoder (::> _ <:: number on the electrically term, white school ...)
①ABC: an input terminal, the input binary code
②Y0 ~ Y7: an output terminal, the output of decimal 10
③G: control whether the chip work
Normal: 100 (G1, G2A, G2B)
Stop: 011 (G1, G2A, G2B)
Textbooks P99 Example Three, to be continued. .