STATUS REGISTER It is a group of flipflops
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STATUS REGISTER It is a group of flip-flops. But it is not used to store data. Ø Each bit in the status register is used to indicate a particular condition of a register after an arithmetic or logic operation. Ø Each bit is called as a flag bit and therefore the status register is also known as “flag register” Ø
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’. q The 4 -bit status register has four bits named as S, Z, P and Cy. q 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 Memory types Volatile Non-volatile RAM SRAM ROM DRAM PROM EPROM
RAM Pronounced “ramm”, acronym for random access memory, Ø A type of computer memory that can be accessed randomly; that is, any byte of memory can be accessed without touching the preceding bytes. Ø RAM is the most common type of memory found in computers and other devices, such as printers. Ø There are two basic types of RAM: Dynamic RAM (DRAM) Static RAM (SRAM) Ø
RAM Two types: Dynamic RAM and Static RAM. Ø The two types differ in the technology they use to hold data, dynamic RAM being the more common type. Ø Dynamic RAM needs to be refreshed thousands of times per second. Ø Static RAM does not need to be refreshed, which makes it faster; but it is also more expensive than dynamic RAM. Ø Both types of RAM are volatile, meaning that they lose their contents when the power is turned off. Ø
ROM Pronounced “rahm”, acronym for read-only memory, computer memory on which data has been prerecorded. Ø Once data has been written onto a ROM chip, it cannot be removed and can only be read. Ø Unlike main memory (RAM), ROM retains its contents even when the computer is turned off. Ø ROM is referred to as being nonvolatile, whereas RAM is volatile. Ø
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