Design a software-based (microprogrammed) control unit for the following architecture. Use the structure shown in Figure 1 which is a slightly modified version of Project2.

You have to design and decide the size of the control memory and microinstruction format in order to control the simple computer. In addition, you might need to change the mapping algorithm as well.

MuxASel MuxAOut 00011011 IROut (0-7)Memory OutputAddress Register OutCOutALU

MuxBSel MuxBOut 00011011 ɸIROut (0-7)Memory OutputOutALU

MuxCSel MuxCOut 01 MuxAOutOutA

Figure 1: Slightly Modified Version of Project 2 (i.e., here MUXC takes input from Address

Register File instead of MUXA)

INSTRUCTION FORMAT

There are two types of instructions as described below.

(1) Instructions with address reference has the format shown in Figure 2:

• The OPCODE is a 5-bit field (See Table 1 for the definition).
• The REGSEL is a 2-bit field (See left side of Table 2 for the definition).
• The ADDRESSING MODE is a 1-bit field (See Table 3 for the definition).
• The ADDRESS is 8 bits

OPCODE

REGSEL

ADDRESSING MODE

ADDRESS

Figure 2: Instructions with an address reference

(2) Instructions without address reference has the format shown in Figure 3:

• The OPCODE is a 5-bit field (See Table 1 for the definition).
• DESTREG is a 3-bit field which specifies the destination register (See right side of Table 2 for the definition).
• SRCREG1 is a 3-bit field which specifies the first source register (See right side of Table 2 for the definition).
• SRCREG2 is a 3-bit field which specifies the second source register (See right side of Table 2 for the definition).
• The least significant two bits are unused and have the value 00.

At most one register from the address register file (AR/SP/PC) can be used on the right hand side of the instructions without address reference. In addition, if one of AR/SP/PC is used on the right hand side, it must be the A input of ALU (i.e., must be supplied by means of MUXC).

OPCODE

DESTREG

SRCREG1

SRCREG2

00

Figure 3: Instructions without an address reference

Table 1: OPCODE field and SYMBols for opertions and their descriptions

Table 2:REGSEL (Left) and DESTREG/SRCREG1/SRCREG2 (Right) select the register of interest for a particular instruction

Table 3: Addressing modes

EXAMPLE

The code given below adds data that are stored at M[A0]+M[A1]+M[A2]+M[A3]+M[A4] and stores the total at M[A5]. It is written as a loop that iterates 5 times.

You have to determine the binary code, write it into memory, and execute all these instructions.

ORG 0x20 # Write the program starting from the address 0x20

LD R0 IM 0x05 # R0 is used for iteration number

LD R1 IM 0x00 # R1 is used to store total

LD R2 IM 0xA0

MOV AR R2 # AR is used to track data adrress: starts from 0xA0

LABEL: LD R2 D # R2 <- M[AR] (AR = 0xA0 to 0xA4)

INC AR AR # AR <- AR + 1 (Next Data)

ADD R1 R1 R2		# R1 <- R1 + R2 (Total = Total + M[AR])
DEC R0 R0		# R0 <- R0 – 1 (Decrement Iteration Counter)
BNE IM LABEL		# Go back to LABEL if Z=0 (Itertaion Counter > 0)
ST R1 D		# M[AR] <- R1 (Store Total at 0xA5)