Remember: 31-26 is always the opcode 25-21 is rs (R or I) 20-16 is rt (R or I) 15-0 is rd/shamt/funct (R) OR immediate (I) 25-0 is address for J type
PCWrite (Write ALU output to PC) and PCWriteCond (Write if ALU’s zero is true)
PCWriteand/orPCWriteCondlets you write ALU output to the PC
For PCWrite, if it’s on then it’ll let PC be written into.
If PCWriteCond is on, then it will let PC write only if the ALU outputs 0
- This is done via subtraction (if two nums subbed = 0, then zero flag is emitted)
- I do NOT know about less than / greater than tho
PCSource (Mux key for PC — math or PC+4)
todo i think it’s 2 bits? It signals to the mux at the far right whether we read the PC resulting from a jump (comes through the shift left 2) or from an ALU operation (ALU result OR ALU out)
IorD (Chooses between PC or ALU out)
- “Instruction or Data”
- Indicates whether memory address is from the PC or from an ALU operation (ALU out I’m guessing)
- Yes it’s ALU out considering the data goes leftwards into the cycle = needs another cycle = must be stored in register
IRWrite (Instruction Register Write)
- Lets the instruction register be filled with a new instruction from memory (goes directory into instruction register)
MemRead and MemWrite
- Lets you read/write into memory / RAM!
- Write data comes from PC or ALU out (remember these are addresses)
MemToReg (The small mux switching from ALU out or Memory data register)
- The register file receives data from memory (Memory Data Register), NOT from the ALU output.
ALUSrcA (1 bit, chooses if A for ALU is “A” from reg, or raw PC)
- I needa remember A and B stuff near the registers again.
ALUSrcB (2 bit. Chooses if B takes from reg, 4, sign-extended immediate, or shift-left-2 immediate)
RegWrite (Sets whether we’re writing into the register)
- We can only write into the register data from memory or from ALU out, so remember that.