Unit – II CMOS

The Role of CMOS CMOS RAM is nothing more than very low power static RAM. Older CMOS RAM devices offered 64 bytes, and later implementations provide an extra 64 bytes. The latest mother boards use 512 bytes or more to store the CMOS setup along with ESCD information needed by the PC’s plug and play system.

Configuring the CMOS setup CMOS must be entered manually through a setup routine. You need to boot the computer from a floppy disk containing the CMOS setup utility. When the setup routine resides on the system, you can usually access the setup during system initialization by pressing one or more keys simultaneously.

Entering CMOS setup You can launch the setup in the first few moments after the system boots – just after the memory test is finished, but before the operating system starts to load. AMI BIOS - Del key during the POST Award BIOS - Ctrl-Alt-Esc DTK BIOS - Esc key during the post IBM PS/@ BIOS - Ctrl-Alt-Esc ALR PC - F2 Compaq PCs - F10 Gateway 2000 PC - F1 Sony PC - F3 CMOS Maintenance and Troubleshooting Typical CMOS related symptoms

The following symptoms highlight many of the most common issues to strike the CMOS RAM. Symptom 1: Changes to CMOS are not saved after rebooting the PC Symptom 2: The system appears to be performing poorly Symptom 3: CMOS mismatch error occurs Symptom 4: Some drives are not detected during boot Symptom 5: The system boots from the hard drive, even though there is a bootable floppy disk in the drive Symptom 6: Power management features are not available Symptom 7: PnP support is not available, or PnP devices do not function properly Symptom 8: devices in some PCI slots are not recognized or not working properly Symptom 9: You cannot enter CMOS setup even though the correct key combination is used Symptom 10: The system crashes or locks up frequently Symptom 11: COM Ports don’t work Symptom 12: The RTC doesn’t keep proper time over a month Symptom 13: The RTC doesn’t keep time while switch power is off Symptom 14: you see an “invalid system configuration Data” error

CMOS Password Troubleshooting Passwords are usually regarded as a necessary evil – a means of keeping out the malicious and the curious. Passwords also cause their share of problems. As systems are passed from person to person or department to department, passwords often become lost or forgotten.

Does anybody know the password? Check with friends, colleagues, supervisors – some one just might know the password. For Award BIOS, you can try BIOSTAR or AWARD_SW

Check for a password clear jumper open the case and take a look at the motherboard. After the system boots, power down again and reset the jumper..

Force a configuration change Try removing a SIMM or DIMM and powering up the PC. Press F1 for setup. This gets you into CMOS, where you can disable the password without clearing the CMOS RAM

Clear the CMOS RAM There are several ways to clear the CMOS. You can restart the PC and reset the jumper CMOS Battery maintenance The backup battery will need to be replaced on a fairly regular basis. Before replacing the battery be sure that you have a valid CMOS backup – either on paper or floppy disk. Turn off power system, unplug the system and remove the battery. Replace the original battery and install the new one according to the system manufacturers instruction. Secure the new battery and restart the system.

CPU Identification and Troubleshooting The central processing unit has become one of the most important developments ever realized in integrated circuit technology although a CPU can handle mathematical calculations, the CPU itself was not designed to handle floating point math as an internal function. In order to deal with high performance floating point math in hardware, a math co-processor or numerical processing unit was developed o work in conjunction with the CPU.

CPU Essentials There are several sets of processor signals that you should be familiar with the data bus, the address bus and the control bus represents as individual control signals.

The Busses The data bus carries the information to and from the CPU, and it is perhaps the most familiar yardstick of CPU performance. The number of wires in the bus represents the number of bits that can be carried at any point in time. Data lines are typically labeled with a “D” prefix. The size of a data bus is typically 8,16,32 or 64 bits. In order for the CPU to read or write data, it must be able to specify the Precise I/O port or location in system memory. “Locations” are defined through the use of an address bus. The number of bits in address bus represent the number of physical locations that the CPU can access.

Processor modes Processors are capable of operating in several different modes. The term “term” refer to the ways in which a processor creates and operating environment for itself. Real Mode

The original IBM PC could address only in 1 MB of RAM. The decision made in those days have carried forward and in each new processor, the processor had to support a mode that would be compatible with the original Intel 8088 chip – this is called real mode

Protected Mode The protected mode has numerous advantages: The protected mode offers full access to all of the system’s memory The protected mode has the ability to multitask, meaning that the operating system can manage the execution of multiple programs simultaneously The protected mode offers support fr virtual memory, which allows the system to use the hard disk o emulate additional system RAM when needed

Virtual Mode It emulates the real mode from with in the protected mode and allows DOS program to run. A protected mode operating system such as windows can actually create multiple virtual real mode machines – through numerous virtual machines.

Modern CPU concepts

CISC Vs. RISC CPU CPU are based on a CISC (complex instruction set computing)architecture. This approach allows any number of instructions to be used in the CPU, and the CPU must provide all of the internal circuitry needed to process each instructions. CISC offers versatility at the expense of CPU performance. CISC CPU are typically found in general purpose desktop and mobile computers.

RISC (reduced Instruction set computing) architecture uses a limited number of very powerful instructions. This CPU type requires fewer transistors in the CPU for processing and generally results in faster CPU performance with far lower power consumption. RISC processors are often less versatile than their CISC counterparts. Processor Speed The processor speed is a function of several critical factors. Speed is largely related to design of the processor circuit itself – the design dictates the internal timing requirements that limit the maximum speed the processor can handle. Speed is also influenced by manufacturing factor such as the circuit size and die size. In general, smaller chips can run faster because of shorter signal runs and lower power consumption.

Processor Power management Processor consume a relatively large amount of power. In order to reduce the PC’s power demands and improve performance the +5 volt operating voltages of years past have given way to processors, support chips and expansion devices that operate at far volts. Since the power consumption of a cpu is related to its processing speed and internal activity, Intel eventually developed power management circuitry that enables processors to conserve power. SIMM circuitry is integrated into physical processor chip but operates independently to control the processor power used based on its activity level.

System Clocks Every modern PC uses multiple system clocks. Each clock runs at a specific frequency – normally measured in MHz. A clock tick is the smaller unit of time in which processing takes place and is some times called a cycle. Some types of processing work can be done in one cycle while others require more cycles. The term system clock generally refers to the speed of the memory bus running on the motherboard and usually not that of the processor. The various clocks in a modern PC are created using a single clock generator circuit to generate main system clock and then various clock multiplier circuits create the other clock signals.

Architecture Performance Features This part describes some of the performance enhancing features found in a modern microprocessor.

Super scalar Architecture Program instructions are processed through circuits called execution units or execution engines. The term super scalar architecture refers to the use of multiple execution units to allow the CPU to process more than one instruction simultaneously with every clock cycle. Pipelining CPU process instructions and generate results through a complex series of transistor switches inside the CPU die itself. Each CPU processed one instruction at a time- that is an instruction was fetched and processed completely, then a new instruction was fetched. The pipelining technique allows a new instruction to start processing while a current instruction is still being processed. This way a CPU can actually work on several instructions during the same clock cycle. Superpipelining By making the pipelining longer each stage performs less work, and the processor can be scaled to a higher clock frequency. Speculative Execution and Branch Prediction Some CPUs have the ability to execute multiple instructions at once. In some cases, not all of the results of the execution will be used, because changes in the program flow may mean that the given instruction should never have been executed in the first place. Even more advanced processors combine this with branch prediction, where the processor can actually predict which way the branch will go based on past history. Branch prediction improves the handling of branches by making use of a special small cache called the branch target buffer. Dynamic(Out of order)Execution The dynamic execution techniques allows the processor to evaluate the program’s flow and choose the best order in which to process instructions. For example instruction2 can be executen before instruction1 has finished. Register Renaming and write buffers Register renaming is the techniques used to support multiple execution paths without conflicts between different execution units trying to use the same registers. Multiprocessing Multiprocessing is the technique of running a system with more than one processor. The idea is that you can double system performance using two processors instead of one. Motherboard Support A motherboard capable of handling multiple processors. This means additional sockets or slots for the extra CPUs and a chipset capable of handling the multiprocessor configuration Processor Support Processors that are suitable for use in a multiprocessor system, not all the processors are suitable and only some versions of the processor are suitable.

Troubleshooting CPU Problems Each CPU such as the 8088 carried only 29000 transistors. When one of those transistors failed, it would usually result in a complete system failure – the PC would crash or freeze entirely. Of course any CPU fault is very serious, but there are now many cases where a system may boot, but crashes when certain specific CPU functions are attempted.

TIPS for Controlling Heat  Use a good quality heat skin/fan that is more than adequately rated for your particular CPU  Use a thin layer of heat sink compound to improve heat transfer between the CPU case and heat sink.  For extremely hot CPU, try a peltier cooler  Select reliable ball bearing type fans with extended services lifetimes.  Fold and tie cable away from areas requiring free air circulations  Use a CPU cooler with an audio alarm system that will alert you in case of either fan malfunction or excessive CPU temperature  Increase air circulation in and out of your computer case by using an auxiliary fan General Symptoms  The system is completely dead  You get a beep code or I/O post code indicating a possible CPU fault  The system boots with no problem, but crushes or freezes when certain applications are run  The system boots with no problem, but crashes or freezes after several minutes of operation  The system BIOS mis identifies the processor  An older system refuses to run properly when the CPU interval cache is enabled  A system malfunctions under HIMEM.SYS after installing anew CPU  The system runs fine, but it reopens the wrong type of CPU  L2 cache fails after upgrading to a Pentium overdrive processor  Some software locks up on systems running 5x86 processors.