Page 67 - DCAP210_INTRODUCTION__TO_MICROPROCESSORS

Unit 4: Microprocessor Architecture

also decides where to keep information in memory and which devices to communicate with by Notes
interfacing with the ALU, memory and input/output devices. The control unit can also shut
down a computer if it or another device, such as the power source, detects abnormal conditions.
4.3.5 Information Exchange
The system bus connects the microprocessor to the peripherals, such as a keyboard, mouse, printer,
scanner, speaker or digital camera. The microprocessor sends and receives data through the system
bus to communicate with the peripherals. It only communicates with one peripheral at a time so
as to not mix up any information and send it to the wrong place. The control unit controls the
timing of the information exchange.
4.4 Microprocessor Memory

The previous section talked about the address and data buses, as well as the RD and WR lines.
These buses and lines connect either to RAM or ROM — generally both. In our sample
microprocessor, we have an address bus 8 bits wide and a data bus 8 bits wide. That means that
8
the microprocessor can address (2 ) 256 bytes of memory, and it can read or write 8 bits of the
memory at a time. Let’s assume that this simple microprocessor has 128 bytes of ROM starting at
address 0 and 128 bytes of RAM starting at address 128.

Figure 4.2: Microprocessor Memory

ROM stands for read-only memory. A ROM chip is programmed with a permanent collection of
pre-set bytes. The address bus tells the ROM chip which byte to get and place on the data bus.
When the RD line changes state, the ROM chip presents the selected byte onto the data bus.

Figure 4.3: ROM Chip

RAM stands for random-access memory. RAM contains bytes of information, and the
microprocessor can read or write to those bytes depending on whether the RD or WR line is
signalled. One problem with today’s RAM chips is that they forget everything once the power
goes off. That is why the computer needs ROM.

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