Can anyone tell me what is wrong with the below statement describing the decode phase of the instruction cycle especially the D7 part?
Now in the decoding phase, the instruction is decoded by the control unit of the CPU as per the operation code. Now the control unit is to decide the type of the instruction. There are 3 types of instructions; memory reference instruction, register reference instruction, and input and output instructions. The opcode can have 8 possible values starting from D0 to D7 or in other words 000 to 111. The type of the instruction is decided by the decoder as per the value of the D7. If the value of D7 is 0, then the instruction type is memory reference type. If the value of D7 is 1, then the instruction type can be either register reference or I/O reference type. At this stage, the CPU refers to the value of the 16th bit of the instruction format. If D7 = 1 and the address mode, I is 1, the instruction type is register reference type, otherwise, when I is 0, the instruction type is I/O reference type. The decode phase of the instruction ends after deciding the type of instruction and subsequent decisions based on the address mode bit. For the memory referencing instruction, the decode operation decides the addressing mode using the I bit as described earlier; 0 for direct addressing and 1 for indirect addressing.
Instruction execution: