LECTURE#11

M. Mateen Yaqoob

The University of Lahore

Spring 2014

DESIGN OF A BASIC COMPUTER 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

CONTROL LOGIC GATES 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

CONTROL OF REGISTER AND MEMORY 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

Control functions can be combined into three Boolean expressions as:

The read operation is recognized by symbol M[AR]

CONTROL OF COMMON BUS 16-bit common bus is controlled by selection inputs S2, S1 and S0.

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:

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:

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

ASSIGNMENT Perform question number 5-15 and 5-16

Due Date: 14-07-2014