Architecture (System Basics)
Address Bus - Address numbers are given to memory locations, each memory location has a unique address number. The CPU must be able to read or write data to these addresses, to do this the CPU accesses the desired memory location by translating the memory address to binary.
Data Bus - Once the memory location has been accessed the data is transferred along the data bus.
. Some of the memroy is in the form of 'ROM' (Read Only Memory). This is a chip with program coding permanently burned into it. Its contents are not lost if the machine power is switched off. Some, or all, of the system/video/disk BIOS is stored in ROM form.
. Information never flows into the CPU from the Address Bus. The Address Bus is onlu used to allow the CPU to access various peripheral chips.
. Since ROM cannot be written to, data only flows on to the Data Bus from the ROM chip - and never in the opposite direction.
. Since RAM can either be read or written, there is a need to allow data to flow between memory and the CPU in either direction (only one direction at a time!).
. the I/O expansion bus also has to be capable of both recveiving and transmitting data on the Data Bus (e.g. a modem has to transfer data in both directions).
Control Bus - This bus transports several control signals. Many CPUs have two major control lines, one that is brought to a low voltage level to indicate tat a read is taking place and one that is brought to a low voltage level to indicate a write taking place. Naturally, only one of these lines can be brought low at any one time. The PC range of processors, the xxx86 range, treats memory and I/O devices differently and therefore has seperate control lines for each.
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