Question

To which pin on the ram chip does the address decoder connect in order to signal

Answer 1

Memory Address Decoding

The processor can usually address a memory space that is much largerthan the memory space covered by an individual memory chip.

 

In order to splice a memory device into the address space of the processor, decoding is necessary.

 

For example, the 8088 issues 20-bit addresses for a total of 1MB of memory address space.

 

However, the BIOS on a 2716 EPROM has only 2KB of memory and 11 address pins.

 

A decoder can be used to decode the additional 9 address pins and allow the EPROM to be placed in any 2KB section of the 1MB address space.

Memory Address Decoding



Memory Address Decoding

To determine the address range that a device is mapped into:



 

This 2KB memory segment maps into the reset location of the 8086/8088 (FFFF0H).

 

NAND gate decoders are not often used.

Rather the 3-to-8 Line Decoder (74LS138) is more common.

Memory Address Decoding

The 3-to-8 Line Decoder (74LS138)



Note that all three Enables (G2A, G2B, and G1) must be active, e.g. low, low and high, respectively.

Each output of the decoder can be attached to an 2764 EPROM ( 8K X 8).

Memory Address Decoding



The EPROMs cover a 64KB section of memory.

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