LC33

1
The LC-3 State Machine
2
Output Forming Logic
Input Forming Logic
Current State
LC-3 Datapath
FF
. . . . .
FF
3
Current State
Datapath Control
LC-3 Datapath
FF
OFL
IFL
. . . . .
FF
Next State
Datapath Status
4
Instruction Fetch Example

• 1. Copy PC contents to MAR
• enaPC 1 ldMAR 1

ldMAR
enaPC
ldPC
PC

• 2. Perform memory read
• selMDR1 ldMDR1

selPC
Increment PC selPC 00 ldPC 1

• 3. Copy memory output register contents to IR
• enaMDR 1 ldIR 1

selMDR
ALU
ldIR
IR
ldMDR
enaMDR
5
Fetch Control Sequence
Current State
Next State
Outputs
enaMDR
enaPC
ldMAR
selPC
ldPC
ldIR
ldMDR
selMDR
6
The Control Logic
Examples IR86 IR20 IR119
BranchTaken
7
The Control Logic
enaMARM
aluControl
enaMDR
memWE
enaALU
selEAB1
selEAB2
selMAR
selMDR
ldMDR
ldMAR
flagWE
regWE
enaPC
selPC
ldPC
ldIR
SR2
SR1
DR
Flip Flops
IFL
OFL
IR
N
Z
P