4917emu

Tell me how a computer works, how a computer works

Like to learn how to CODE and program? Then this is for you. This emulator is based on the
4917 microprocessor (a hypothetical designed just for a computer course named COMP1917 at the
UNSW University of New South Wales) which was used to teach microprocessor basics.
It is 4bit, has 16 memory locations and 4 registers: Instruction Pointer (IP), Instruction Store (IS),
General Register 0 (R0) and General Register 1 (R1). Each memory location can store a
number between 0 and 15, and there are 16 instructions.

 

This is how it works: There is an an Input Memory where you enter your program and
the program is stored. There is also an Output where you can see the output of your program.
And there is a Control center where you can control your program like run it, step through, debugg it, etc.

Here it is on an iPad:

 

 

App Usage

Just enter your own program or use a predefined one and hit Step or Run.

There is a Extended Instruction Code set. For example, you can turn the front LED on and off.  See below for more details.

For example the following 4917 machine code program would print 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15.

8 0 3 11 1 15 0

Introduction

So what is a microprocessor? It basically is there and just waits. For what? For your input. There is on input, output and control "thing".

• On the Input section, you enter values right into the memory of the microprocessor. You program your own program here or load one from the disk storage.
• On the Control section you control what happens next. Run your program, inspect memory, etc.
• On the Output you can see what happens. Read printed values, read dumped memory, etc.

There is one more thing, the Instruction Pointer (IP), the Instruction Set (IS) and the Registers (R0 and R1).

• IP (Instruction Pointer): This is just a value that points to the memory cell which will be executed next. A zero means, please execute the code within memory location zero next time you start the machine by hitting step or run.
• IS (Instruction Set): This is the fetched instructions where the IP points to. In our example you can see that the IP is 0 and the IS is 8 and 0. This is because of a two byte command 8 and 0.
• R0 (Register 0): This is just a temporary internal microprocessor storage.
• R1 (Register 1): This is an other one. Sure you can read values from and write values to a register.

Instruction Codes

1-byte Instructions

(Note: In the 4917 each ‘byte’ is 4 bits, so a byte is just a number between 0 and 15.)

0 = Halt

1 = Add (R0 = R0 + R1)

2 = Subtract (R0 = R0 – R1)

3 = Increment R0 (R0 = R0 + 1)

4 = Increment R1 (R1 = R1 + 1)

5 = Decrement R0 (R0 = R0 – 1)

6 = Decrement R1 (R1 = R1 – 1)

7 = Ring Bell

2-byte Instructions, value of the second byte is called <data>

8 = Print <data> (The numerical value of <data> is printed)

9 = Load value at address <data> into R0

10 = Load value at address <data> into R1

11 = Store R0 into address <data>

12 = Store R1 into address <data>

13 = Jump to address <data>

14 = Jump to address <data> if R0 == 0

15 = Jump to address <data> if R0 != 0

 

Customer Review

Version 2.3.1

- A complete new redesign for iOS 7 and iOS 6

- iPad: fixed app crash

Version 2.3

Germany

Makes... ★★★★★

by Danksa Grabowski - Version 2.3 - Mar 13, 2013

...a lot of fun! 😀 But it sucks that it sometimes cleares the memory without a reason...

Version 2.1

USA

This is a great app. ★★★★★

by BoxerBomb - Version 2.1 - Nov 18, 2012

This app does just what it says. Very little memory locations which limits what you can make but great for learning just the app I was looking for. Things to add: More memory locations SAVING and an better info dialog

 

Support

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