Question

We wish to add the instruction jal (jump and link). Make any necessary changes to the datapath or to the control signals if needed. You can photocopy figures to make it faster to show the additions. How many product terms are required in a PLA that implements the control for the single-cycle datapath for jal?

Short answer

The datapath changes are made by changing PC+4 as input to MemToReg multiplexer and changing it to 2-bit control signal. The control signals are changed by expanding two multiplexers. Four product terms are required in a PLA to implement the control cycle for single-cycle datapath for jal.

Step-by-step solution

Define jal instruction

Jump and link (jal) instruction is used for procedure calling. A program counter (PC) contains the address of the instruction that will be executed next. The jal instruction jumps to the jump address and save the address of the next instruction to $ra or $31. The format of the instruction is

jal address

Make required changes in datapath to add jal instruction

Thejalinstruction performs the following steps:

• It takes the value of Mem[PC] and stores it in Instruction word.
• The value of PC+4 is stored in R[31]
• The Jump address is stored in PC.

The datapath is changed by making PC+4 as input to MemToReg Multiplexer. The MemToReg signal is changes to 2-bit control signal.

Make required changes in control signals to add jal instruction

The changes are required to made in control signals to add jal instruction. Two multiplixer are expanded. A value 31 is added to the multiplexer controlled by RegDst. And multiplexer controlled by MemToReg is expanded to have PC+4 as input. The address control lines for jal are set as follows:

Instruction	RegDst	ALUSrc	MemtoReg	Reg Write	Mem Read	Mem Write	Branch	ALUOp1	ALUOp0	Jump
R-type	01	0	00	1	0	0	0	1	0	0
lw	00	1	01	1	1	0	0	0	0	0
sw	xx	1	xx	0	0	1	0	0	0	0
beq	xx	0	xx	0	0	0	1	0	1	0
J	xx	x	xx	0	0	0	x	x	x	1
JAL	10	x	10	1	0	0	x	x	x	1

The number of product terms for required in PLA are 4 to add control for single cycle data path for jal instruction.