Proj 6. Using Jasmin to Run x86 Assembly Code (15 pts)

From: https://samsclass.info/126/proj/126p6jas.htm

What You Need for This Project

Any computer, running any OS

Purpose

To practice writing and running basic x86 assembly code, using the Jasmin interpreter.

Install Java

Open a Web browser and go to

Click the "Do I have Java?" link. Follow the instructions on your screen to download and run a Java applet. If you don't have Java, download and install it.

Note: If you are using the Mac, you should use Safari, not Chrome.

Download Jasmin

In a Web browser, download this file:

jasmin-1.5.8-PC.jar

Understanding the Jasmin Window

Double-click the Jasmin-1.5.8-PC.jar file you downloaded. Jasmin launches.

Click the "New File" button. Look over the window, referring to the diagram below:

Find and examine these sections:

Registers

Data used during processing is stored in the registers EAX, EBX, ECX, and EDX.

The ESP (Extended Stack Pointer) contains the address of the top of the Stack.

The EIP (Extended Instruction Pointer) contains the address of the the next instruction to be processed.

Flags

These one-bit values that are used for branching. For example the JZ instruction will jump if the Zero flag is 1 (set), and the JNZ instruction will jump if the Zero flag is 0 (cleared).

Code

This is where you type in commands, such as mov eax,4

Help

Help messages appear here.

Memory

This processor has 0x1000 = 4096 bytes of RAM, which is not enough to run complete modern programs, but plenty for running little assembly programs for learning purposes.

With the Memory pane scrolled to the top, as shown in the image above, you see memory that the program will use to store data during processing.

Scroll this pane to the bottom to see the Stack, which starts at address 0x1000 and grows downward.

Using mov Instructions

In the Code section, type in these instructions.

mov eax, 4
mov ebx, 6

These instructions move the number 4 into eax, and the number 6 into ebx.

At the top of the Jasmin window, click the green Run button, as shown below.

The program runs. When it stops, notice these things, as shown below:

EAX contains 4
EBX contains 6
EIP contains 2, because instructions 0 and 1 have been executed
In the Code area, instruction 2 is highlighted in green, indicating that it is the next instruction to be processed.

Troubleshooting

If you make an error in an instruction, the program will stop prematurely. Fix the instruction, and click the Reset button. Then you can run it again.

Storing Results in Memory

Add more lines to your Code section to make your program look like this:

mov eax, 4
mov ebx, 6
mov [eax], ebx
mov ecx, eax
add ecx, ebx
mov [eax+4], ecx

Here's what these instructions do:

`mov eax, 4`	Move the value 4 into eax
`mov ebx, 6`	Move the value 6 into ebx
`mov [eax], ebx`	Move the value in ebx (which is 6) into the memory location pointed to by eax (memory location 4)
`mov ecx, eax`	Move the value in eax (which is 4) into ecx
`add ecx, ebx`	Add the value in ebx (which is 6) to the value in ecx (which is 4), and put the result into ecx (the result is 10)
`mov [eax+4], ecx`	Move the value in ecx (which is 10) into the memory location four past the location pointed to by eax (memory location 8)

Run the program. When it completes, you should see these results, as shown below:

EAX = 4
EBX = 6
ECX = 10
Memory location 0x4 contains 6
Memory location 0x8 contains 10 (A in hexadecimal)

Using the Stack

In Jasmin, click File, New.

In the Code section, type in these instructions.

mov eax, 4
mov ebx, 6
push eax
push ebx

Before running the program, notice the ESP: it contains 256, as shown below

256 is 0x100 in hexadecimal--this is where the Stack ends.

Scroll down in the Memory pane to see the last values. As show below, the last location is at 0xFC. This value is 32 bits long, so it contains four bytes, at locations 0xFC, 0xFD, 0xFE, and 0xFF. The ESP points to the next byte, 0x100.

Understanding Push

At the top of the Jasmin window, click the green Run button.

These instructions move the number 4 into eax, and the number 6 into ebx. Then both values are pushed onto the stack.

Notice these things, as shown below:

EAX contains 4
EBX contains 6
ESP contains 248, which is 0xF8, the new top of the stack.
Memory location 0xFC contains 4, the first value pushed onto the stack.
Memory location 0xF8 contains 6, the second value pushed onto the stack.

Understanding Pop

Add a pop instruction to your code, so it now looks like this:

mov eax, 4
mov ebx, 6
push eax
push ebx
pop ecx

Run the code.

Notice these things, as shown below:

ECX contains 6, the value popped off the top of the stack.
ESP contains 252, which is 0xFC, the new top of the stack.
Memory location 0xFC contains 4, the first value pushed onto the stack.
Memory location 0xF8 contains 6, which is now the top value on the stack.

Reversing a Sequence

In Jasmin, click File, New.

In the Code section, type in these instructions.

mov eax, 1
mov ebx, 2
mov ecx, 3
mov edx, 4
push eax
push ebx
push ecx
push edx
pop eax
pop ebx
pop ecx
pop edx

These instructions load values into the four registers, push them onto the stack in order, and pop them off the stack in order

However, since the stack is a FILO (First In, Last Out) structure, this reverses the order of the values.

Push the Step four times to execute only the first four instructions, as shown below:

You see the values 1, 2, 3, and 4 loaded into the EAX, EBX, ECX, and EDX registers, as shown below.

Push the Step four more times to execute only the next four instructions.

You see the values 1, 2, 3, and 4 pushed onto the stack, as shown below.

Push the Step four more times to execute the remaining four instructions.

Now the registers contain these values:

EAX = 4
EBX = 3
ECX = 2
EDX = 1

as shown below.

6.1 Find the Value (15 pts)

In Jasmin, click File, New.

In the Code section, type in these instructions.

mov eax, 1
mov ebx, 9
mov ecx, 49
push eax
push ebx
push eax
push ecx
push ebx
push eax

pop eax
pop ebx
pop ecx
add eax, ebx
pop ecx
add eax, ecx
pop ebx
pop ecx
add eax, ebx

Load and run this program. Find the value of eax, which is covered by a green box in the image below. Enter that value into the form below.

6.1: Recording Your Success (10 pts)

Use the form below to record your score in Canvas.

Name or Email:

Value:

Posted 9-3-18