Microprocessor Architecture: ROM and EPROM

Memory Types

• ROM (Read Only Memory): data can only be read, no writing is allowed; used to store permanent programs; nonvolatile type of memory
• EPROM (Erasable Programmable Read Only Memory): programmable by user, information can be erased by exposing to ultraviolet light, and reprogrammed using EPROM programmer; disadvantage: all data gets erased even if only one bit needs to be changed
• EEPROM (Electrically Erasable Programmable Read Only Memory): similar to EPROM, but erasing is done by electrical signals; advantage: memory location can be selectively erased and reprogrammed; disadvantage: manufacturing process is complex and expensive
• R/W Memory (Read/Write Memory): also called RAM, allows reading and writing data, volatile type of memory; used to check execution of programs

R/W Memory Subtypes

• SRAM (Static RAM): consists of flip-flops, bit status remains until next write operation or power supply is switched off; advantages: fast access time, no refreshing required; disadvantages: low package density, expensive
• DRAM (Dynamic RAM): stores data in capacitors, requires refreshing to maintain charge, high package density, low cost; disadvantage: requires refreshing circuit every few milliseconds

Microprocessor Clock

• Also called clock rate, measures speed at which microprocessor executes instructions
• Measured in megahertz (MHz) or gigahertz (GHz)
• CPU requires a fixed number of clock ticks to execute each instruction
• Faster clock speed means more instructions can be executed per second

Classification of Memory

• ROM (Read Only Memory): data can only be read, no writing allowed; used to store permanent programs; nonvolatile
• Masked ROM: program or data permanently installed at time of manufacturing; cannot be altered; expensive but economic for large quantities
• PROM (Programmable Read Only Memory): similar to masked ROM, but has fuse links; can be programmed, but disadvantage: no chance of re-growing fuse, and programmed data can be altered due to aging

I/O Devices and Interfacing

• MPU communicates with outside world through I/O devices
• Two methods of I/O device identification and communication: Direct I/O (Peripheral) and Memory-Mapped I/O
• Direct I/O uses two instructions (IN & OUT) for data transfer, MPU uses 8 address lines to send I/O device address, and can differentiate between input and output devices using control signals IOR and IOW
• Memory-Mapped I/O uses 16 address lines to identify I/O device, similar to communicating with memory location