Topic 10

The 6502 microprocessor was used in many early personal computers and gaming systems, such as the Atari, Apple II, Nintendo, and Commodore 64. There were 65 different instructions in the 6502's instruction set. Most of these these instruction, such as the "Add with Carry" (ADC) instruction, supported various addressing modes. For example, the instruction,

ADC #$42

uses "immediate: addressing and adds the number 42 to the current contents of the accumulator (i.e. the 42 is stored as part of the instruction).

Alternatively, the instruction

ADC $42

uses "absolute" addressing and, thus, adds the contents found at absolute memory location 42 to the current contents of the accumulator.

The difference in these assembly instructions is the inclusion of the '#' sign. When the '#' is included, the corresponding op code is '0x69' (hex), while the op code for the absolute addressing ADC instruction is '0x6D'. Hence, these are actually two different instructions in the 6502 instruction set.

The total number of instructions (i.e different op codes) in the 6502 instruction set is 156.

In Section 10.7, we gave an example of the Instruction Register (IR) and Decoder for the ADD instruction, with 000 op code, using the following Figure,

Submit a brief description of the changes you would make to the design in the previous Figure to handle the 156 instructions in the 6502 instruction set (as opposed to the eight instructions in the RISC202-8 instruction set in the notes).