Question

What is the difference between ROM and RAM in construction i.e., though both are used for memory storage what is the main thing that makes one of them permanent and the other one temporary?

Answer 1

ROM is constructed by arranging array of switches -
you can read the contents at output-data port, by putting address on read-address port.
Data is hardwired during manufacturing(mask programmed) itself. you can hardwire the data during design phase aswell... but nobody does that due to security reasons. if it helps you can think of ROM as a pre-recorded casette.

RAM is built using latches-
you can write data into them by placing address on write-address port, and data on input-data port.
you can read data from RAM by placing address on read-address port, and reading data on output-data port.

RAM : once the power is off, you are not sure what data will appear on latches, when you turn it on again.
ROM : once the power is off, since the data is hardwired, the physical structure remains. so data is permanent. to erase the data in a ROM, you need to physically rip the structure of switch !!