Computer Architecture Fundamentals: CPU Modes, Timing, and Bus Synchronization

Protected Mode in x86 CPUs

Protected Mode is an operational mode of x86-compatible CPUs from the 80286 series and beyond. It introduces several new features designed to improve multitasking and system stability. These include memory protection, hardware support for virtual memory, and task-switching capabilities. Protected Mode also provides hardware support to interrupt a running program and change the execution context to another, enabling pre-emptive multitasking.

Virtual 8086 Mode and the 80386DX

The 80386DX, introduced in 1985, incorporated significant advancements. This microprocessor featured a 32-bit data bus and could address up to 4 GB of memory. Unlike the 80286, which used fixed-size memory segments, the 80386 allowed for the definition of variable-sized memory segments, greatly enhancing memory management flexibility.

The 80386DX maintained the three primary operational modes: Real Mode, Protected Mode, and Virtual 8086 Mode. Key differences in the 80386 included:

• A specific flag for Virtual 8086 Mode.
• 2 bits for page entry.
• 6 new instructions.
• 32-bit control registers.

Timing and Switching Circuits in Digital Systems

Timing and switching circuits form the fundamental basis of any device designed to select or change memory-controlled signals. Their applications span various fields, including telecommunications switching, satellite transitions, and the operation of digital computers.

Families of digital logic devices perform a variety of logic functions through components known as logic gates, such as OR, AND, and NOT gates. Common logic families include:

• Transistor-Transistor Logic (TTL)
• Complementary Metal-Oxide-Semiconductor (CMOS) Logic: Known for very low power consumption, though often with slightly lower operating speeds.

Other varieties of logic circuits exist, such as Resistor-Transistor Logic (RTL) and Emitter-Coupled Logic (ECL), which are used for specific speed requirements, including very high-speed applications.

Understanding the System Clock and Processor Speed

Processor speed is primarily affected by the system clock. The system clock is an electronic pulse used to synchronize all processing operations within a computer.

Clock speed is measured in:

• Megahertz (MHz): Where one MHz equals one million cycles per second.
• Gigahertz (GHz): Where one GHz equals one billion cycles per second.

Bus Synchronization and Wait States

A common challenge in computer systems is matching the speed of the bus cycles with that of connected expansion cards. It is common for an expansion card to operate slower than the system bus.

To manage this discrepancy, a "ready" signal is utilized. If a card is not ready to complete a data transfer, it can signal the bus to:

• Slow down its operation.
• Insert "wait states" until the card synchronizes with the bus cycle and is ready for the next operation.