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M. Mateen Yaqoob The University of Lahore Spring 2014.

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1 M. Mateen Yaqoob The University of Lahore Spring 2014

2  The basic computer consists of: 1. Memory unit with 4096 (4KB) words of 16 bits each 2. Nine registers: AR, PC, DR, AC, IR, TR, OUTR, INPR and SC 3. Seven flip-flops: I, S, E, R, IEN, FGI and FGO 4. Two decoders: 3x8 operation decoder and 4x16 timing decoder 5. 16-bit common bus 6. Control logic gates 7. Adder and logic circuit connected to the input of AC  The memory unit is a standard component that can be obtained readily from a commercial source

3  Input comes from the two decoders and bits 0 through 11 of IR  The other inputs are;  AC bits 0 through 15 to check if AC=0 and to detect the sign bit in AC(15)  DR bits 0 through 15 to check if DR=0  the values of seven flip-flops  The outputs of control logic circuit are:  Signals to control the input of nine registers  Signals to control the read and write inputs of memory  Signals to set, clear, or complement the flip-flops  Signals for S2, S1 and S0 to select a register for the bus  Signals to control the AC adder and logic circuit

4  Control inputs of registers are LD (load), INR (increment), and CLR (clear)  First three statements specify transfer of information from a register or memory to AR  The content of source register or memory is placed on bus and content of bus is transferred into AR by enabling its LD control input  Fourth statement clears AR to 0  Last statement increments AR by 1


6  Control functions can be combined into three Boolean expressions as:  The read operation is recognized by symbol  M[AR]


8  16-bit common bus is controlled by selection inputs S2, S1 and S0.

9  Each binary number is associated with a Boolean variable X1 to X7, corresponding to gate structure that must be active in order to select register or memory for bus  When X1-1 the value of S2S1S0 must be 001 and output of AR will be selected for the bus  Table 5-7 is recognized as truth table for binary encoder  Boolean functions for encoder are:

10  To determine the logic for each encoder input, it is essential to find the control function which place the corresponding register onto the bus  For example, to find the logic that makes x1=1, we scan all register transfer statements and extract those statements that have AR as source  Boolean function of X1 is:

11  The logic gate that generates X7 must also be applied to read input of memory  So, the Boolean function for X7 is same as the one derived previously for read operation  In similar manner we can determine logic gate for other registers

12  Perform question number 5-15 and 5-16  Due Date: 14-07-2014

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