Registers  Flip-flops are available in a variety of configurations. A simple one with two independent D flip-flops with clear and preset signals is illustrated on the following slide. Although packaged together, the two flip-flops are unrelated. The second arrangement shows an octal flip-flop in which the eight D flip-flops are not only missing the Q’ and preset lines, but all the clock lines are ganged together and driven by pin 11 so the flip- flops are loaded on the rising transition.

Registers

 All eight clear signals are also ganged, so when pin 1 goes to 0, all the flip-flops are forced to their 0 state.  While one reason, for ganging the clock and clear lines is to save pins, the chip in this case is used in a different way from eight unrelated flip-flops. It is used as a single 8-bit register.  We can also use two such chips as a 16-bit register by tying their respective pins 1 and 11 together.

Memory Organization  To build large memories a different organization is required, one in which individual words can be addressed. One such organization that meets this criterion is shown on the next slide. This example shows a memory with four 3-bit words. Each operation reads or writes a full 3-bit word. The organization requires fewer pins then an octal flip-flop and extends easily to large memories. The memory has eight input lines and three output lines.

Memory Organization

 We could have designed a circuit in which the three OR lines were just fed into the the three output lines, but in practice the same lines are used for both input and output.  What is needed is an electronic switch that can make or break a connection in a few nanoseconds. Such a switch is called a noninverting buffer. It has a data in, data out, and a control line. When control is high, the buffer acts like a wire. When control is low, the buffer acts like an open circuit.

Memory Organization

 An inverting buffer acts like a normal buffer when control is high, and disconnects the output from the circuit when control is low.  Both kinds of buffers are tri-state devices, because they can output 0, 1, or none of the above.  Buffers also amplify signals so they can drive many output simultaneously. They are sometimes used in circuits for this reason, even when their switching properties are not needed.

Memory Chips  The memory shown previously easily extends to larger sizes.  For example, to extend to 4 words of 8 bits each we add five more columns of four flip- flops each, as well as five more input and output lines. To extend to eight words of three bits each, we add four more rows of three flip- flops each and one more address line. For maximum efficiency, the number of words in memory should be a power of 2, but the size of words can be anything.

Memory Chips  For any given memory size, there are various ways of organizing the chip.  A 4-Mbit chip could be organized as 512K words of 8 bits each or 4096K words of 1 bit each.  Aside - on some pins a high voltage causes an action to happen while on others a low voltage causes the action. Thus, we will say that a signal is asserted or negated to avoid this issue. A signal S is asserted high, but S’ is asserted low.

Memory Chips  Since a computer will generally have multiple memory chips, a signal is needed to select the chip that is currently needed. The CS’ (Chip Select) signal is provided for this purpose.  We also a need a signal to distinguish reads from writes - the WE’ (Write Enable) signal.  The OE’ (Output Enable) signal is asserted to drive the output signals. When it is not asserted, the chip output is disconnected.

Memory Chips

 An alternative addressing scheme is used in the second chip organization of the previous slide.  To address the chip, first a row is selected by putting its 11-bit number on the address pins. Then the RAS’ (Row Address Strobe) is asserted.  Then a column number is put on the address pins and CAS’ (Column Address Strobe) is asserted. A single bit is read or written. This reduces the number of pins required, but we need to clock cycles to address memory.

RAMs and ROMs  The memories we have seen can be both read and written. Such memories are called RAMs (Random Access Memories).  RAMs come in two varieties, static and dynamic.  Static RAMs (SRAMs) are constructed using circuits similar to the D flip-flop. They retain memory as long as power is on, are fast, and are often used to implement level 2 cache memory.

RAMs and ROMs  Dynamic RAMs (DRAMs) do not use flip- flops. A DRAM is an array of cells, each cell contains one transistor and one capacitor. The capacitors can be charged or discharged, allowing data to be stored.  Since the electric charge tends to leak away, every bit in a DRAM must be refreshed every few milliseconds. DRAMs have a high capacity, thus they are almost always used for main memory. On the other hand, they are slow.

RAMs and ROMs  An FPM (Fast Page Mode) DRAM is organized as a matrix of bits and requires a row and then a column address to be presented.  FPM DRAM is gradually being replaced by EDO (Extended Data Output) DRAM, which allows a second memory reference to begin before the previous memory reference completes. Both FPM and EDO chips are asynchronous (the address and data lines are not driven by a single clock).

RAMs and ROMs  SDRAM (Synchronous DRAM) is a hybrid of static and dynamic RAM and is driven by a single synchronous clock. It is often used in large caches.  ROMs (Read-Only Memories) have their data inserted during manufacture. The only way to replace the program in a ROM is to replace the chip.  The PROM (Programmable ROM) can be programmed (once) in the field by selectively blowing fuses.

RAMs and ROMs  The EPROM (Erasable PROM) can be field- erased as well by exposing the EPROM to a strong ultraviolet light for 15 minutes.  The EEPROM can be erased by applying pulses to it rather than UV light. It can also be programmed in place (an EPROM requires a special programming device). EEPROMs are slow compared to DRAMs and SRAMs. Flash memory is block erasable and rewritable. They are often used in digital cameras, but they wear out after about 10,000 erasures.

RAMs and ROMs

CPU Chips  All modern CPUs are contained on a single chip.  Each CPU chip has a set of pins, through which all communications with the outside world occur.  The pins on a CPU chip can be divided into three types: address data control

CPU Chips  These pins are connected to similar pins on the memory and I/O chips via a collection of parallel wires called a bus.  To fetch an instruction from memory, the CPU: puts the memory address of the instruction on its address pins asserts one or more control lines to inform the memory that it wants to read a word waits for a signal from the memory that it has put the data on the CPU’s data pins accepts the word and carries out the instruction

CPU Chips  Two of the key parameters that determine the performance of a CPU are the number of address pins and the number of data pins.  A chip with m address pins can address up to 2 m memory locations.  A chip with n data pins can read or write an n- bit word in a single operation. A CPU with 8 data pins will take four operations to read a 32-bit word, whereas one with 32 data pins takes just one operation. More pins implies more expensive, however.

CPU Chips