MEMORY INFORMATIONS FOR MICROPROCESSORS

In theMicroprocessor an 8-bit address bus and a data bus 8 bits wide. This means that the microprocessor can handle (28) 256 bytes of memory, and it can read or write 8 bits of memory at once. Assume that this simple microprocessor, has 128 bytes of ROM starting at address 0 and 128 bytes of RAM starting at address 128.

ROM stands for read-only memory. A ROM chip is programmed with a permanent collection of preset bytes. The address bus indicates the ROM chip which byte to get and set the data bus. When the RD line changes state, the ROM chip presents the selected byte data bus.
using microprocessor memory chip with the primary function. Learn more about the memory of the microprocessor, ROM, BIOS and what is the boot sector.
RAM chip

RAM is random access memory. RAM contains bytes of information, and the microprocessor can read or write bytes depending on whether the RD or WR line is signaled. A problem with the RAM chips now that they forget everything when the power is off. This is why the computer needs ROM.

Incidentally, nearly all computers contain some amount of ROM (it is possible to create a simple computer that contains no RAM - many microcontrollers do this by placing a handful of bytes of RAM on the processor chip in its itself - but in general, impossible to create one that contains no ROM). In a PC, the ROM is called the BIOS (BASIC Input / Output System). When the microprocessor starts, it begins executing instructions it finds in the BIOS. The BIOS instructions do things like test the hardware on the machine, and then it goes on the hard disk to fetch the boot sector (see How Hard Drives Work for details). The boot sector is another small program, and the BIOS store it in RAM after reading it from disk. The microprocessor then begins executing instructions from the boot sector of RAM. The boot sector program will tell the microprocessor to getting something else from the hard disk into RAM, then runs to the microprocessor, and more. Thus, the microprocessor loads and executes the entire system.