Instruction Format

Instruction Format

All instructions are 32 bits long. There are four types of instruction format.

• Arithmetic instruction format
• Conditional Branch  and Immediate format
• Unconditional Jump format
• Input and Output instruction format

Arithmetic instruction format

2 bits      6 bits           4 bits       4 bits      4 bits      12 bits

00

OPCODE

S-reg

S-reg

D-reg      000

The first two bits are always 00, indicating that the instruction is an Arithmetic or Register transfer type of instruction. S-reg is the source register. D-reg is the destination register. The last 12 bits are always 0, as they are not used.

Conditional Branch and Immediate format

2 bits      6 bits           4 bits       4 bits       16 bits

01

OPCODE

B-reg

D-reg

Address

The first two bits are always 01, indicating that the instruction is a Conditional Branch and Immediate type of instruction. B-reg is the base register. D-reg is the destination register. The last 16 bits may be an address or an immediate data.

• When the last 16 bits contain data, the D-reg is always 0000.
• The Address may at times be treated as data, which is direct addressing.
• An indirect Address is calculated as :

  Effective Address = Content (B-reg) + Address

• Conditional Branch checks for B and D reg to cause a branch, to a specified Address, or not

Unconditional Jump format 

2 bits      6 bits                           24 bits

10

OPCODE

Address

The first two bits are always 10, indicating that the instruction is an Unconditional Jump type of instruction, with a jump to the specified Address.

 

 

 

Input and Output instruction format

2 bits      6 bits            4 bits      4 bit               16 bits

11

OPCODE

Reg 1

Reg 2

Address

The first two bits are always 11, indicating that the instruction is an Input and Output type of instruction.

• The instruction may read the content of Address/Reg 2 into Reg 1.
• The instruction may write the content of Reg 1 into a specified Address/Reg 2.  

Registers 

There are 16 registers; each of 32 bit long.

• Reg-0 (0000) being the Accumulator.
• Reg-1(0001) being the Zero register, which contains the value 0.
• All other registers are general purpose register.

Buffers

• Input buffer – containing data read by the program
• Output buffer – containing data produced by the program
• Temp buffer – area in memory to store/retrieve the data temporarily.