Day 7 Flashcards
Describe the data path
Registers are connected to the ALU to form a data-path, over which the data flow. The basic operation of the data path consists of selecting one or two registers, having the ALU operate on them, and stroing the result in some register.
Describe the control unit
The control unit is responsible for fetching instructions from main memory and determining their type. The arithmetic logic unit performs operations such as addition and Boolean AND needed to carry out the instructions.
What is a bus?
A bus is a collection of parallel
wires used to connect the components of a computer.
Describe the idea behind the latch?
- latch is a combinational circuit used to “save” memory.*
- It is composed of two NOR gates, inputs : S-set, R-reset, Q and Q’.*
- while S=R=0, the values of Q and Q’ are saved.*
- If S=1 R=0 -> Q=1 Q’=0*
- If S=0 R=1 -> Q=0 Q’=1*
- If S=R=1 we have a problem - inconsistency.*
- We use a clock to control the sensitivity of the circuit to S and R.*
Describe the idea behind flip flops
Flip Flops are used when we want to have the circuit sensitive only at the shift of the clocks from 1 to 0 or from 0 to 1.
It uses the delay of the inverter in order to do so.
Describe the order of operations on A and B
On which input(A or B) does a one-regiter-operation being executed?
- A comes first. For instance, Sub -> A-B
- one-register-operation is done on B
Which instructions are being executed for each instruction in T0-T2?
What should you write on test?
- fetch & execute
- On test, T0-T2: fetch
Note: refering to memory is done using a bus different from that of the CPU, thus we can perform two operations simultaneuosly.
Mention a very important point about A
A has no output. It is used merely to save the value of some register before having a computation to be stored in C. C of course has an output.
Solve “swap Ra,Rb”
Solve “shc Ra,Rb,Rc”
using only A,C
Recall: MA,MD are not used for temporary computations.
Just recall
What are the 11 control signals?
C=B, INC4, AND, OR, ADD, NOT, SUB, SHR, SHL, SHC, SHRA
When do we use wait?
after reading from memory, we need to wait for the operation to be completed.
- Explain ld control signal.
- save on n last 5 bits of some register.
- Decr - n=n-1
- Goto6 - repeat T6 until n = 0.
What are combinational circuits?
A circuit with multiple inputs and multiple outputs in which the outputs are uniquely determined by the current inputs. Note: not all circuits act this way; circuits containing memory elements may well generate outputs that depend on stored values as well as the input variables.