CSC231 Bash Tutorial 5

--D. Thiebaut (talk) 08:04, 3 March 2017 (EST)

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Testing Instructions

Writing a short assembly program to answer quick questions is simple and can be quick. We'll do a couple examples.

Testing how DIV works

• Get a copy of test_skel.asm

getcopy test_skel.asm

• Its code is shown below for completeness:

;;; Testing sandbox
	        extern  _printDec
	        extern  _printString
	        extern  _println
	        extern  _getInput
		extern	_printRegs
	
	section	.data
a		dd	100
b		dd	33
c		dd	0
ans		dd	0
	
		section	.text
		global	_start
_start: ;-------------------------------------------------

	
;;; exit -------------------------------------------------
		mov	ebx, 0
		mov	eax, 1
		int     0x80

• It has everything needed to test how short pieces of code work.
• Put the following code between the dashed lines:

 		mov	edx, 0
		mov	eax, dword[a]
 		div	dword[b]
 
 		call	_printRegs

• (If the lines are already there, but commented out, just remove the semi-colons.)
• See what the program does. What quantity will be divided by what other quantity.
• Assembly, link and run:

 nasm -f elf test_skel.asm
 ld -melf_i386 -o test_skel test_skel.o 231Lib.o
 ./test_skel 

• Do you see the quotient and remainder in the registers? Does it make sense?

You should practice doing this simple exercise every time you are not sure about a particular set of instructions. If you start with a skeleton program, such as test_skel.asm, all you have to do is fill in the middle part, assemble, link and run, and you will have your answer.

Hex, Binary, and 2's Complement

• You can use the same method for testing number representations.
• Go to this URL http://www.nasm.us/doc/nasmdoc3.html, and locate Section 3.4.1. Look at the instructions and the different ways Nasm accepts literals.
• Nasm accepts numbers in decimal, hex, binary, octal, and 2's complements.
• Let's try a few:
• Edit your test_skel.asm program and add the following lines between the dashed lines:

		mov	eax, 0x0000FFFF
		mov	ebx, 00000000111111110000000011111111b
		mov	ecx, 1111_1111_1111_1111_1111_1111_1111_1111b
		call	_printRegs

• Assemble, Link and run. You should see eax, ebx, and ecx, as hex, unsigned, and signed (2's complement) integers.

Bash Loops

Bash supports loops! It is a powerful way of repeating commands.

• Try this at the Bash prompt:

 for i in 1 2 3 4 5 6 7 8 9 10 ; do
      echo "i = $i"
 done

• Explanations:

• i is used as the index of the loop. It is declared by putting it right after the for keyword.
• i will take all the values in whatever list is given after the in keyword.
• the semi-colon ends the declaration, and is followed by do
• the next lines will be repeated for every value i will take. In our case, just the echo command, which is Bash's print command.
• we close the body of the for-loop with done

Note that when you want to use the loop variable inside the body of the loop, you write $i. Variables are declared without $ but are prefixed with $ when used.

Several Different Kinds of Loops

• Try these different variations of loops. They contain new features and constructs, and you will figure out how they work by playing with them.

Loop 2

for i in `seq 1 10` ; do
    echo "i is $i"
done

Loop 3

for i in `seq 1 10` ; do
    echo "i is $i"
done

Loop 4

for i in `seq 1 10` ; do echo "i is $i" ;  done

• Loops can be written on one line only. The semi-colon marks the end of commands. Make sure you put spaces around the semi-colons!

Challenge #1:

Use the man page for seq and figure out a way to make your loop print all the numbers from 20 down to 1.