Branch on Equat
ICS53 – MIPS Instruction Set Architecture & Single-Cycle Datapath (continued)
Ex: beq $t0, $s0, label # if Reg[$t0] == Reg[$s0], then PC = PC + 4 + label
• These instructions take two operands from registers and
perform an ALU comparison operation on them, if the operation is “true” then the PC is replaced with the location of the next instruction to execute.
• The label value in the immediate field from the instruction, is not the 32-bit address of the label. Instead the field represents +/- the number of instructions from the PC+4 address.
o Most branch statements are local jumps for loops and conditional checks. These jumps are not to far of places in memory. Therefore, +/- 215 instructions is more than enough range.
o Note, how it is the number of instructions, not the number of bytes. Since all label addresses are multiples of 4, we can increase the range by eliminating the need to store the LSB 00 in the immediate field. Instead we shift the immediate field to the left by 2 to multiple the value by 4.
o Note, the calculation of the relative position of the branch label is performed based on the PC+4 value, not the current instruction. The reason for this will become clear in the next module. For now, just accept this fact.
Branch Fetch – Fetch Instruction
Decode – Register Read (rs: $s0, rt: $t0), Sign Extend Immediate Value
Execute – Compare operands by Performing subtract ALU operation (Reg[$s0] – Reg[$t0])
Execute – If “true” (Zero ==1), set PC value to target address
• In the execute stage, we subtract the register values and check to see if the result is 0 using the Zero output of the ALU. Remember, the Zero output has a value of 1 if the result of the ALU is 0, 0 otherwise.
• While the operands are being compared, the target address (the location of the next instruction to execute if branch is “true”) is calculated by:
o Sign-extend the immediate value (positive or negative 2’s complement value) o Shift left 2 places (multiply by 4 to get word address)
o AddvaluetoPC+4value
• Note that for the branch operation, we use the PCBranch adder to calculate the branch address.
o This is because the entire datapath must execute in a single clock cycle. This means that each component of the datapath can only perform 1 operation. Hardware cannot be reused in the same
instruction during the clock cycle.
UCI ICS 51 – Introduction to Computer Organization Copyright 2020 – Prof. Jennifer Wong-Ma This content is protected and may not be shared uploaded or distributed.
Branch on Equal Instruction Walk-through
Jump
Ex: j label # PC = PC+4[31:28],label[25:26] comma represent concatenation
• Jump instructions perform an unconditional change to the PC based on the value in the instruction
• Jump is different than a branch in the sense that we ARE NOT modifying the PC with respect to how many
instructions forward and backward. A portion of the absolute address to jump to is provided in the addr
field.
• The full 32-bit address cannot be stored in the instruction itself (need 6 bits for opcode, it won’t fit!).
Therefore, we can specify at most 26-bits of the jump address.
o All instructions must be stored on word boundaries of memory (multiple of 4). Therefore, the 2 least
significant bits of the address are always going to be 00.
o We can use this to our advantage and not store these zeros in the instruction (waste of space, also
allows for larger range of addresses to be represented/jumped to). So, if we specify the 26 bits in the
instruction and then shift left by 2 (multiply by 4), we now have 28-bits of the jump address.
o We are still missing 4 bits. Where can we get these? Since when we jump, we are most likely jump
to a location close to the current instruction, then we can take these bits from the current PC (same region of memory)
This means when you use a jump instruction you can only jump to an address in the same section of memory. The upper 4 bits of the PC divide the memory into 16 different sections. Jump instructions only allows you to move to any address within your current section. To jump to another section, you need to use jr (jump to register)
• jr jumps to the full 32-bit address stored in the rs register
o The jump address is the top 4 bits of current PC+4, specifically PC[31…28] appended to the 28-bits
(jump address in instruction shifted left by 2).
• To summarizes, the destination address for a jump instruction is the concatenation of: o upper 4 bits of the current PC + 4
UCI ICS 51 – Introduction to Computer Organization Copyright 2020 – Prof. Jennifer Wong-Ma This content is protected and may not be shared uploaded or distributed.
Jump Instruction Walk-through
o 26-bit address field in instruction
o 00 as the 2 low-order bits (word address, not byte. Therefore, multiply by 4.)
Control Unit
• The names of the control signals are named to represent flags, eg. Branch = 1 means branch is true/taken, Branch = 0 means branch is false/not taken.
o A signal is asserted = 1 or deasserted = 0
• The complete Control Table the 5 basic operations implemented in the
datapath is shown below:
Single-Cycle Control Unit
Instr lw
Opcode
100011
RegDst
0
RegWrite
1
ALUSrc
1
MemRead
1
MemWrite
0
MemtoReg
1
Branch
0
ALUOp
00 (add)
Jump
0
sw
101011
X
0
1
0
1
X
0
00 (add)
0
R-type
000000
1
1
0
0
0
0
0
10 (func)
0
beq
000100
X
0
0
0
0
X
1
01 (sub)
0
jump
000010
X
0
X
0
0
X
X
XX
1
• Note the ALUOp column of the table. This value is a 2-bit control determined within the Control Unit and is used to create the 3-bit ALUControl output for the ALU.
• Because the ALU control is based on not only the opcode, but the function field (R-type), the control logic unit is split into 2 steps.
o This reduces the number of inputs for the ALUOperation Boolean expression from 12-bits (6 for opcode and 6 for func field) to 8-bits (2 for intermediate ALUOp and 6 for func field)
UCI ICS 51 – Introduction to Computer Organization Copyright 2020 – Prof. Jennifer Wong-Ma This content is protected and may not be shared uploaded or distributed.
• Based on the opcode a 2-bit control value, ALUOp, can be created as an intermediate output. This value then combined with the function field produces the proper ALU Operation control.
Opcode Funct field
Control Logic
. Control values for . each Component
ALUOp
ALU Control
ALU Operation
ALUOp
Meaning
00
add
01
subtract
10
look at
funct
11
not used
• When the opcode is lw or sw, ALUOp is set to 00 to denote addition is performed. For beq, ALUOp is set to 01 to denote subtraction is performed. The function field in these cases is ignored (aka Don’t care), as the Immediate instruction format do not contain a function field.
• In the case of R-type instructions, the function field is the unique identifier for the operation (remember that the opcode is set to all zeros in the case of R-type instructions).
o The ALUOp is set to 10, to denote the function field should be considered when determining the ALUControl.
• !! In all of the discussions/practice problems/notes we use the 2-bit ALUOp to denote the operation performed by the ALU unit instead of the 3-bit ALUControl (less to remember).
UCI ICS 51 – Introduction to Computer Organization Copyright 2020 – Prof. Jennifer Wong-Ma This content is protected and may not be shared uploaded or distributed.
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