Lab 4 - Simulator

Introduction

As discussed in class, register-transfer level design can be described as

1. clock signal has a rising edge
2. register outputs change
3. logic works, which can be thought of as arbitrary acyclic code that assigns each variable only once
4. logic results in register inputs changing
5. repeat

In this lab, and the subsequent homework assignment, you will expand on these basic pieces to build your own machine simulator and write binary code for it.

Simulator Skeleton Code

For this lab, we’re providing starter code for a basic simulator in both python (sim_base.py) and java (SimBase.java). Pick one of the two to work with (whichever language you find more comfortable).

The program works from a command-line interface, expecting memory byte values (either directly or in a file) as a command-line argument:

## runs SimBase in java with 8 bytes of memory set
javac SimBase.java
java SimBase 01 23 45 67 89 ab cd ef

## runs SimBase in python with 8 bytes of memory set
python3 sim_base.py 01 23 45 67 89 ab cd ef

## runs SimBase using the contents of programs/halt to set memory
python3 sim_base.py programs/halt
java SimBase programs/halt

Memory contents must be specified in hexadecimal bytes, separated by whitespace.

Working on your own laptop

If you already have either a Java or Python environment on your laptop, you are welcome and encouraged to work on this lab locally.
You may download the starter code, unzip and open the simulator code in your chosen environment.

Working on the portal

If you’d prefer to work in the portal environment, then you can get a copy of the starter code from our course directory by cd-ing into the directory you wish to use and then issuing the following command to unzip the code there:

unzip /p/cso1/lab4.zip

It should create a lab4-simulator directory with all the starter code to open in vim, nano, or your chosen command-line editor.

Hint: remember that if you write Java on the portal, you will need to compile your Java before running it. Once you’ve edited SimBase.java, compile the code with javac SimBase.java before running as described above.

Write the simulator functionality

Each file begins with a function or method named execute which is given two arguments (the current instruction in ir and the PC of this instruction in oldPC) and returns one value (the pc to execute next). The skeleton code just returns oldPC + 1.

Each file also has two global values you can access: R, an array of 4 register values, and M, an array of 256 memory values.

We also provide a get_bits helper function for getting a range of bits from a number which you may use if you wish.

Add code to execute (do not edit other parts of the file) to do the following:

Separate the instruction into parts

Treat the instruction as having four parts:

bits	name	meaning
7	reserved	If set, an invalid instruction. Do not do work or advance the PC if this bit is 1.
[4, 7)	icode	Specifies what action to take
[2, 4)	a	The index of a register
[0, 2)	b	The index of another register, or details about icode

Execute the instruction

(You will probably need to be familiar with the Language nuances of your implementation language of choice before starting on this section.)

Change execute to do different things for different icodes, as follows:

• If reserved is 1, set the next PC to the current PC instead of advancing it and do nothing else.

• Otherwise, see the following table, where rA means “the value stored in register number a” and rB means “the value stored in register number b.” Unless a different value of the pc is specified in the table, also add one to the pc for each instruction.

icode Behaviors: for each icode (3-bit) value, which operation to perform given the operands.

icode	operation
0	rA = rB
1	rA &= rB
2	rA += rB
3	do different things for different values of b: b action 0 rA = ~rA 1 rA = !rA 2 rA = -rA 3 rA = pc
4	rA = read from memory at address rB
5	write rA to memory at address rB
6	do different things for different values of b: b action 0 rA = read from memory at pc + 1 1 rA &= read from memory at pc + 1 2 rA += read from memory at pc + 1 3 rA = read from memory at the address stored at pc + 1 In all 4 cases, increase pc by 2, not 1, at the end of this instruction
7	Compare rA (as an 8-bit 2’s-complement number) to 0; - if rA <= 0, set pc = rB - otherwise, increment pc like normal.

Test your simulator

See Example programs below for suggestions.

Language nuances

Python’s syntax for !x is not x instead.

Java treats bytes (like R[i]) as signed integers, not unsigned. That means they are not good indices (e.g., M[R[i]] might throw an exception if R[i] is negative). However, R[i] & 0xFF treats it as unsigned instead, so M[R[i] & 0xFF] should work.

Python treats bytes as unsigned, so R[i] <= 0 acts like R[i] == 0, which is not what we want. It can be re-written to work correctly as R[i] == 0 or R[i] >= 0x80

Example programs

Halt

The code

00 00 00 80

should advance to pc 3 and then stay there.

Try finding other code that will also do that.

Move

Consider the following code

Bytes	Activity
68 23	move 0x23 into register 2
06	move from register 2 to register 1
60 20	move 0x20 into register 0
54	move from register 1 to memory at address from register 0 (i.e., move 0x23 to address 0x20)
80	halt

If your simulator works, then 68 23 06 60 20 54 80 should end with registers being [20, 23, 23, 00] and a 0x23 in memory at address 0x20.

Try adding code that will read from memory into register 3.

Math

Consider the following code

Bytes	Activity
68 10	move 0x10 into \(R_2\)
42	move value from address \(R_2\) into \(R_0\)
68 11	move 0x11 into \(R_2\)
46	move value from address \(R_2\) into \(R_1\)
24	\(R_1\) += \(R_0\)
68 12	move 0x12 into \(R_2\)
56	move \(R_1\) into address \(R_2\)
80	halt
five 00s	(padding)
23 A1	some numbers to add

If your simulator works, you should end up with 0xC4 (i.e., 0x23 + 0xA1) in address 0x12.

Try adding code that can sum more numbers from memory, not just those two.

Conditional jump

The following should not jump, so three steps should end up with the PC at address 5, not 20:

pseudocode	parts	bytes
\(R_0\) = 10	(6, 0, 0) 10	60 0A
\(R_1\) = 20	(6, 1, 0) 20	64 14
if \(R_0\) <= 0, jump to \(R_1\)	(7, 0, 1)	71

The following should jump, so three steps should end with the PC at address 20, not 5:

pseudocode	parts	bytes
\(R_0\) = −10	(6, 0, 0) −10	60 F6
\(R_1\) = 20	(6, 1, 0) 20	64 14
if \(R_0\) <= 0, jump to \(R_1\)	(7, 0, 1)	71

Read from immediate address

The following should load 20 into \(R_1\)

pseudocode	parts	bytes
\(R_2\) = 10	(6, 2, 0) 10	68 0A
\(R_3\) = 20	(6, 3, 0) 20	6C 14
write \(R_3\) to address \(R_2\)	(4, 3, 2)	5E
read address 10 into \(R_1\)	(6, 1, 3) 10	67 0A

You could also do this without using instruction 4 by setting enough memory that there was already data in address 10:

pseudocode	parts	bytes
read address 10 into \(R_1\)	(6, 1, 3) 10	67 0A
intialize address 10 with 20	0s, then 20	00 00 00 00 00 00 00 14

More, less-explained examples

icode = 0

First load a value into \(R_1\) then set \(R_2\) to equal \(R_1\): 64 14 09

icode = 1

First load a value into \(R_1\) and \(R_2\) then and them together: 64 14 68 20 19

icode = 2

First load a value into \(R_1\) and \(R_2\) then add them together: 64 14 68 20 29

icode = 3

First load a value into \(R_1\) and then do something to it:

• flip all bits: 64 89 34 (result: 76)
• perform !: 64 89 35 (result: 00; try also 35 by itself to result in 01)
• negate: 64 89 36 (result: 77)
• replace with the PC: 64 89 37 (result: 02)

icode = 4

First load a value into \(R_1\) and then load that memory address into \(R_2\): 64 02 49

icode = 5

First load a value into \(R_1\) and \(R_2\) and then store \(R_1\) into memory at \(R_2\): 64 02 68 20 56

icode = 6

First load a value into \(R_1\) and then use an immediate:

• replace with immediate: 64 14 64 20 (result: 20)
• and immediate: 64 14 65 24 (result: 04)
• add immediate: 64 14 66 20 (result: 34)
• replace with memory contents at immediate: 64 14 67 02 (result: 67)

icode = 7

First load an address into \(R_1\), a value into \(R_2\), and then conditionally jump:

• jumps: 64 14 68 ff 79
• jumps: 64 14 68 80 79
• jumps: 64 14 68 00 79
• does not jump: 64 14 68 01 79
• does not jump: 64 14 68 7f 79

Check-off

To check-off this lab, show a TA your simulator and the memory contents that do the tasks listed above.