STATUS REGISTER It is a group of flip-flops. But it is not used to store data. Each bit in the status register is used to indicate a particular condition of a register after an arithmetic or logic operation. Each bit is called as a flag bit and therefore the status register is also known as “flag register”
STATUS REGISTER • It reflects the result obtained in the ALU. For example, if the result is negative, the sign bit is set to ‘1’ and if the result is positive, the sign bit is reset to ‘0’. • The 4-bit status register has four bits named as S, Z, P and Cy. • S-bit stands for sign. This bit set if the MSB of the result is 1, it is cleared if the MSB of the result is 0.
STATUS REGISTER • Z-bit stands for zero. This bit set if the result in the ALU contains all ‘0’s, it is cleared if the result is not equal to zero. • P-bit stands for parity. The bit can be set for odd parity or even parity. • Cy-bit stands for the carry bit. The arithmetic circuit produces Cyout when the result has exceeded the 4-bit limit. This bit is reset when there is not Cyout.
Memory Characteristics and Functions • Random Access Memory - RAM • Temporary and volatile • Can be read or written • Read Only Memory - ROM • Permanent and non-volatile • Can only be read
Semiconductor Memorytypes Volatile Non-volatile RAM ROM SRAM DRAM PROM EPROM
RAM • Pronounced “ramm”, acronym for random access memory, • A type of computer memory that can be accessed randomly; that is, any byte of memory can be accessed without touching the preceding bytes. • RAM is the most common type of memory found in computers and other devices, such as printers. • There are two basic types of RAM: Dynamic RAM (DRAM) Static RAM (SRAM)
RAM • Two types: Dynamic RAM and Static RAM. • The two types differ in the technology they use to hold data, dynamic RAM being the more common type. • Dynamic RAM needs to be refreshed thousands of times per second. • Static RAM does not need to be refreshed, which makes it faster; but it is also more expensive than dynamic RAM. • Both types of RAM are volatile, meaning that they lose their contents when the power is turned off.
ROM • Pronounced “rahm”, acronym for read-only memory, computer memory on which data has been prerecorded. • Once data has been written onto a ROM chip, it cannot be removed and can only be read. • Unlike main memory (RAM), ROM retains its contents even when the computer is turned off. • ROM is referred to as being nonvolatile, whereas RAM is volatile.
Types of ROM • 1. Programmable Read Only Memory (PROM) • Empty of data when manufactured • May be permanently programmed by the user • 2. Erasable Programmable Read Only Memory (EPROM) • Can be programmed, erased and reprogrammed • The EPROM chip has a small window on top allowing it to be erased by shining ultra-violet light on it • After reprogramming the window is covered to prevent new contents being erased • Access time is around 45 – 90 nanoseconds • 3. Electrically Erasable Programmable Read Only Memory (EEPROM) • Reprogrammed electrically without using ultraviolet light • Must be removed from the computer and placed in a special machine to do this • Access times between 45 and 200 nanoseconds