Building a Simple Terminal Demo in Assembly Language for Linux

Crafting a ‘demo’ program A ‘walk-through’ of the program development cycle for an example in assembly language

Our purpose • We want to illustrate the steps that a Linux program needs to take when modifying the normal ‘canonical mode’ terminal behavior • We want to write it in assembly language • Our Project #2 involves something similar • Here we want to ‘Keep It Simple’ (KISS) • But yet we want to show the essentials • We might see new Pentium instructions

Just a tiny change • Users can normally ‘cancel’ a program • They can do it by typing <CONTROL>-C • It’s important for stopping “infinite loops” • The system sends a ‘termination’ signal • This avoids the need for a system ‘reboot’ • But we can ‘reprogram’ this tty capability • We just turn off a bit in the ‘c_lflag’ field

Our ‘nocbreak.s’ demo • Step 1: get the terminal’s initial settings • Step 2: save a copy of these settings • Step 3: modify the ISIG bit in ‘c_lflag’ field • Step 4: install the ‘modified’ tty settings • Step 5: let user do some keyboard input • Step 6: reinstall original terminal settings • Step 7: Quit (i.e., return control to Linux)

Step 1: Get ‘tty’ settings • We can use the ‘tcgetattr()’ function • It’s part of the system’s runtime library • Use ‘man’ command to see how it’s called • Here’s its function prototype: int tcgetattr( int fileno, struct termios &tty ); • We can call it using assembly language: • Push the arguments (in right-to-left order) • Call the function: call tcgetattr • Discard the arguments from the stack

Here’s the code .section .data ttywrk: .space 60 # for ‘termios’ object .section .text pushl $ttywrk # push the address pushl $0 # push device-ID call tcgetattr # call runtime library addl $8, %esp # discard arguments

Step 2: copy the object • We can setup a loop to perfortm copying • Loop can copy structure one byte at a time • Total number of bytes is loop-count (60) • Put source-address into a cpu register • Put dest’n-address into a cpu register • Advance addresses as each byte is copied • Use ‘loop’ opcode to decrement-and-jump

Here’s the data .section .data ttysav: .space 60 # original structure ttywrk: .space 60 # our working copy

And here’s the code .section .text movl $ttywrk, %esi # setup source addr movl $ttysav, %edi # setup dest’n addr movl $60, %ecx # setup loop-count nxmv: # label the loop-body movb (%esi), %al # copy src byte to AL movb %al, (%edi) # copy AL to dest’n incl %esi # advance src-addr incl %edi # advance dst-addr loop nxmv # finish coping bytes

Step 3: modify the flag-bit • We know where the ‘c_lflag’ field is • It’s starts 12 bytes into ‘termios’ structure • We got this info from our ‘ttyinfo.cpp’ demo • Similarly we can find that ISIG bit is bit #1 • We want to “reset” this bit (i.e.,clear it to 0) • We could use a bitwise AND operation • But Pentium offers us another way (BTR)

Here’s the code .equ ISIG, 0 # symbolic constant .section .data ttywrk: .space 60 # for termios object .section .text movl $12, (%edx) # offset for ‘c_lflag’ btr #ISIG, ttywrk(%edx) # resets bit #1

Brief digression • Other Pentium bit-manipulations: BTS (bit-set) BTR (bit-reset) BTC (bit-complement) BT (bit-test) • These operations all have this “side effect”: • the previous bit-value gets transferred to the CF-bit (Carry Flag) within the Pentium’s EFLAGS register • Why? So you can use JC (or JNC) afterward

Step 4: Install new behavior • We can use the ‘tcsetattr()’ function • Use ‘man tcsetattr’ to see how its called • Requires three function arguments: • Device’s ID-number (i.e., 0 for keyboard) • A flag-value, to specify buffer-flushing • The address of the new ‘termios’ object • As usual, these arguments have to be pushed in reverse (i.e., right-to-left) order

Here’s the function-call .section .text pushl $ttywrk # address of the object pushl $TCSAFLUSH # flag-value pushl $0 # keyboard’s device-ID call tcsetattr # call to runtime library addl $12, %esp # rebalance stack # NOTE: Similar code is used later in step 6

Step 5: Try new tty behavior • We want to let the user type some input • In particular, we want to test <CTRL>-C • We’ve changed the normal tty handling • Prove <CTRL>-C won’t stop the program • Find out what the new response will be • We need program to ‘read’ from keyboard • Can use ‘read()’ from the runtime library

How ‘read()’ works • Function’s prototype shows 3 arguments: • Device ID-number (e.g., 0 for the keyboard) • Address for an input-buffer (we create buffer) • Maximum number of bytes that will be read • In canonical mode, the ‘read()’ call won’t return until either the user hits <ENTER> or the maximum number of bytes have been transferred into the input-buffer

So here’s the ‘read()’ call .section .data inchar: .space 1 # room for 1 byte .section .text pushl $1 # maximum bytes pushl $inchar # buffer’s address pushl $0 # keyboard’ ID call read # call to C library addl $12, %esp # discard arguments

Testing for <EACAPE>-code • We needed a way to stop the program • Can’t quit by using <CONTROL>-C now • Our solution: quit by hitting <ESCAPE> • So program needs to test for its ascii-code • ASCII-code for ESCAPE-key equals 0x1B • Our loop includes a compare-and-branch

Testing for the ‘exit’ condition .section .data inchar: .space 1 # buffer for user input .section .text again: … cmpb $0x1B, inchar # user typed ESC? jne again # no, reenter loop # otherwise, fall through to next instruction

A ‘tweak’ for esthetics • When we tested our ‘nocbreak’ demo, we did not like the screen’s appearance • Our program’s final output was ‘garbled’ by the subsequent command-shell prompt • We wanted to make the output prettier • So we added a additional code-fragment • A ‘newline’ control-code gets printed after each keypress by the user (using ‘write()’)

In-class exercises • Programmers can choose among several ways of accomplishing a particular task • Example: there’s more than one way to copy a 60-byte data-structure from one place in memory to another • We don’t have to do it one-byte-at-a-time • We don’t have to use both %esi and %edi • Try doing the copying in some other ways

Using a common array-index • Here’s an idea for a different copying scheme .section .text xorl %esi, %esi # array-index movl $30, %ecx # word-count nxwm: movw ttywrk(%esi), %ax # fetch word movw %ax, ttysav(%esi) # store word addl $2, %esi # next word index loop nxwm

Using a ‘scaled’ array-index # we use a ‘scaled index’ to do array-addressing .section .text xorl %esi, %esi # clear to zero movl $30, %ecx # loop-count nxwd: movw ttywrk( , %esi, 2), %ax movw %ax, ttysav( , %esi, 2) incl %esi # increment index loop nxwd:

Exercise • Try to devise the ‘most efficient’ method you can think of for copying the 60-bytes • But what does ‘most efficient’ mean? • Using the fewest assembly statements? • Using the fewest cpu regisers? • Executing the fewest loop-iterations? • Will your “solution” be the same no matter what you think “most efficient” means?

Building a Simple Terminal Demo in Assembly Language for Linux