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.In this case the hex number is first converted toan 8-bit binary number.This process is not performed by the50An introduction to logic gates and their usesFigure 5.2The truth table fora NOT gatemicroprocessor but by other external circuits.By the time it reachesthe microprocessor it has been converted to the binary equivalent of101001002.The NOT gate has only one input so, to handle an 8-bit binary word,we will need eight NOT gates.Now it becomes much easier.EachNOT gate inverts just one of the bits and all the outputs are groupedtogether to form a new hex number.See how it works in Figure 5.3.The result was the hex number 5BH.This is curious.If we add A4H tothis result of 5BH we get FFH or all  ones in binary, 111111112.Therewas nothing special about the number A4H.It happens with any pairof numbers generated by NOT gates.Why is this? Figure 5.3 gives aclue.Figure 5.3Inverting a hexnumber51Introduction to Microprocessors and Microcontrollerslike someone reading the back of your newspaper as you are readingthe front  except that registers don t find it irritating. Port , by the way,is just a fancy electronic word meaning  connection.Transistors,generally, have three wires going to them and so are described asthree-port devices.The integer unit handles all instructions like integer arithmetic bitmanipulation and transferring data to and from the external memoryand is organized into a three-stage pipeline.In Figure 13.3, the secondclock pulse executes the first instruction.The next clock pulseexecutes the second instruction and the last clock pulse executes thethird.We have achieved the target of one clock pulse per clock pulse.And in the fourth clock pulse, we can see the next instruction justarriving to be decoded immediately after the first write-back.Figure 13.3Integer unitpipelineFloating-point unitThis has a further 32 registers but in this case, they are 64-bits wideand to fill a register with a single clock pulse, there is an internal 64-bitbus connecting it with the cache.The pipeline is five stage: prefetch,buffer, decode, execution and write-back.Memory bufferThis acts as a buffer for the external memory.The buffers include tworeads and three writes, each up to 32 bytes.It is also used in writing-back to the cache.Big and little endiansThe main memory is divided into locations each having its ownaddress.Each location can hold a single byte of information.If wewanted to store a 32-bit number, then we would have to utilize fourconsecutive locations.188The PowerPCImagine that we wished to store the 32-bit number00000000 01010101 00010001 111111112 and we had addresses24646603H, 24646602H, 24646601H and 24646600H available.Little-endian format would store the most significant byte in thehighest memory address so, in our example, the data 00000000 wouldgo into address 24646603H.This is used by Intel microprocessors.Big-endian, which Motorola uses, works the other way around.Themost significant byte is put in the lowest memory address so, in ourexample, the data 00000000 would go into address 24646600H.These are shown in Figure 13.4.All the PowerPC microprocessors areswitchable to enable little or big-endian to be used.Figure 13.4Big and littleendiansPowerPC 970A large number of PowerPCs have continued to power the Apple-Macand IBM desktops and, in addition, support both the UNIX and Linuxoperating systems.The latest offering is the 970 with its 52 million transistors started lifeas a 1.8 GHz device and has now progressed to 2.0 GHz.This mayappear slow but it has compensating attributes such as its 900 MHzbus as opposed to the 533 MHz bus of the Pentium 4.It is a 64-bit micro so it handles data in 64-bit chunks but remainscompatible with earlier 32-bit designs.It has two level 1 caches, onefor instructions at 64 kB and a data cache of 32 kB, which aresomewhat larger that the Intel product but both companies use a level2 cache of 512 kB.As memory size is continuing to increase with each design, the sizeof memory that can be directly accessed increases with the move to64-bit processing.The Pentium 4 can access 40 GB of memory,which seems excessively large at the moment but there was a timewhen 4 MB was something to wonder at.The PowerPC 970 canhandle memory of Star Trek proportions measured in terabytes(thousands of Gigs) [ Pobierz całość w formacie PDF ]