Verilog code for Microcontroller (Part 1 - Specification)

In this project, Verilog code for a microcontroller is presented. Before implementing, the instruction set architecture for the microcontroller is presented in this part. (Part-2 and Part-3)

Instruction Set Architecture:

Each instruction is 12 bits. There are 3 types of instructions by encoding, shown as following:

• M type: one operand is accumulator (sometimes ignored) and the other operand is from data memory; the result can be stored into accumulator or the data memory entry (same entry as the second operand).
• I type: one operand is accumulator and the other operand is immediate number encoded in instruction; the result is stored into accumulator.
• S type: special instruction, no operand required. (e.g. NOP)

The instruction encoding space is shown in the following table.

Code space (binary)	Code space (hex)	Type	Number of Instructions	Note
0000_0000_0000 - 0000_1111_1111	000-0FF	special instructions  (S type)	256	Currently only NOP used,255 free slots
0001_0000_0000 - 0001_1111_1111	100-1FF	unconditional jump (I type)	1	GOTO
0010_0000_0000 - 0011_1111_1111	200-3FF	ALU instructions (M type)	32	16 instructions, 2 destination choices
0100_0000_0000 - 0111_1111_1111	400-7FF	conditional jump (I type)	4	JZ, JC, JS, JC
1000_0000_0000 - 1111_1111_1111	800-FFF	ALU instructions (I type)	8	Currently 7 used, 1 free slot

These instructions can be grouped into 4 categories by function.

1. ALU instruction: using ALU to compute result;

2. Unconditional branch: the GOTO instruction;

3. Conditional branch: the JZ, JC, JS, JO instruction;

4. Special instruction: the NOP.

M type instructions

The general format of M type instruction is shown as following.

The following table contains the detailed information of each M type instruction. Note that “aaaa” encodes the 4 bit address of data memory, and the “d” bit means destination of the result, i.e. if d = 1, result is written to Acc, otherwise the result is written to the same memory location as the operand

Note that all M type instructions are ALU instructions.

Comments:

1. For circulative shift, only the operand from memory entry is rotated, i.e. flags are not involved. For example, if 8’b1000_0000 is shifted left circulative for 1 bit, it becomes 8’b0000_0001, and no flag is affected.

2. For logical shift instructions, 0 is always filled in, and C flag is set to the last bit that is shifted out. For example, if 8’b1000_0000 is shifted left logically for 1 bit, it becomes 8’b0000_0000, and C = 1. Another example, if 8’b0000_0100 is shifted right logically for 3 bit, it becomes 8’b0000_0000, and C = 1. If no bit is shifted out, Z flag and C flag are not affected.

3. For arithmetic shift right instructions, the MSB is filled in, and C flag is set to the last bit that is shifted out. For example, if 8’b1000_0001 is shifted right arithmetically for 1 bit, it becomes 8’b1100_0000, and C = 1. Another example, if 8’b0000_0100 is shifted right arithmetically for 3 bit, it becomes 8’b0000_0000, and C = 1. If no bit is shifted out, Z flag and C flag are not affected.

I type instructions

The general format of M type instruction is shown as following.

The following table contains the detailed information of each M type instruction.

Note that I type instructions contains unconditional branch, conditional branch, and ALU instructions.

S type instructions

The general format of S type instruction is shown as following.

There is only one S type instruction, i.e. the NOP instruction.

Sample Testing Program

After completion the hardware implementation of the MCU, you need to write testbench and load instructions to verify the design by simulation. Below is some testing programs to verify the MCU.

Test1: Program loading, accumulator, and memory loading, GOTO

Test2: Addition and subtraction (M type)

Test3: Logic operation (M type)

Test4: Addition, subtraction, logic operation (I type)

Part 2 will be the data-path and control unit design for the micro-controller.

Part 3 will be the Verilog code for the whole microcontroller.

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