Computer Systems Register Notation

1. Comp201 Computer SystemsRegister Notation

2. Register Notation • Used to represent the data transfers between the parts of the CPU during the execution of an assembly language instruction • Register is represented by its initials (e.g. PC, R0 etc) • The contents of a register are shown by enclosing the register symbol in square brackets [PC] • The operation of data transfer is shown by <- • e.g. [$3] <- [$3] + [$4] • This would be interpreted as “take the contents of $3, and add the contents of $4, placing the result in $3”

3. More detail about timing: • Each CPU operation is carried out under the control of the controller which can: • make a register output a value onto a bus (eg1. PC_out)(eg2. a_out, sel_a = 4) • set up the ALU or memory interface to carry out a particular function (func = add) • make a register read a value from a bus (eg1. $5)(eg2. C_in sel_c = 5) • For instance, to add the contents of $4 to the PC, storing the result back in the PC on the 3 bus architecture :PC_out, a_out, sel_a = 4, func = add, alu_out, PC_inand wait sufficient time for the transfer to take place - usually a clock cycle

4. Control Step • Each control step takes one clock cycle Source registers output here Destination registers read values here Clock cycle Signals must stabilise before clock Signals propagate through system

5. C Bus B Bus Memory A C A MI TEMP PC IR Reg File (R0-R15) ALU B B A bus Showing the data paths from the previous example, • PC_out, sel_a = 4, a_outt, func = add, alu_out, PC_in

6. Z 1 Z R1 D Q R2 R2 D Q clk clk 1 0 1 R1in R1out R2in R2out R1out, R2in clk Timing Example: Register Register RTL: [R2] <- [R1] actions: R1 output R2 input

7. R3 Another example: Register ALU Register time to settle clk: outputs enabled inputs read RTL: R1 Q clk 3 [R3] <- [R1] + [R2] R3 D Q R1in R1out 0 clk R2 actions: Q func clk 5 R1out, R2out, add, R3in R3out R3in R2in R2out

8. 3 R3 R1 R3 R2 D D D Q Q Q 8 5 Eg. - Register ->ALU -> Register Z clk: clk RTL: 3 [R3] <- [R1] + [R2] R1in R1out clk 0 actions: 8 clk func R1out, R2out, add, R3in 5 R3out R3in R2in R2out Z

9. The execution of an assembly language instruction may require a sequence of such steps e.g. to fetch an instruction [IR] <- M[[PC]] [PC] <- [PC] + [1] Or, to perform a jal command, [PC] <- [PC] + [1] (if not already done) [ra] <- [PC] [PC]<- location of call Or, to perform a lw command, [temp] <- [$base] + disp [$dest] <- M[temp] . 1 2 Imm_16_out Imm_20_out sign_extend mem_write mem_read temp_out alu_func temp_in alu_out pc_out b_out pc_in a_out sel_a sel_b sel_c ir_in c_in Register Transfer Notation X X X X X X

10. More about RTL descriptions • Register transfer language lets you define computer operations unambiguously in an "algebra-like" notation. • The RTL expression [$3] means the contents of register 3 (i.e., whatever is in register 3). • The RTL expression [M(1234)] means the contents of memory whose location (i.e., address) is 1234. Or, sometimes expressed as 0x1234 to denote hex. • The RTL expression <-  means "is copied to" (from right to left). • You cannot write the expression $3 = 4 in RTL, as it would mean that register 3 is the same as the number 4. The correct expression is [$3] = 4 which means that register 3 holds the value 4. The RTL expression D3 <- 4 is also meaningless. The expression should be [D3] <- 4.

11. In the tables in the back of the WRAMP manual, the instructions are shown in RTL notation, e.g.: sub Rd, Rs, Rt sle Rd, Rs, Rt beqz Rs, offfset jal address Rd <- Rs - Rt Rd <- Rs< Rt if (Rs=0) PC<-PC+offset $ra<-PC, PC<-Address WRAMP Instructions in Register Notation