View source for SPO600 64-bit Assembly Language Lab

[[Category:SPO600 Labs]][[Category:Assembly Language]]
{{Admon/lab|Purpose of this Lab|In this lab, you will experiment with assembler on the x86_64 and aarch64 platforms.}}
{{Admon/tip|SPO600 Servers|Perform this lab on [[SPO600 Servers]] (you may use your own x86_64 system if desired, along with the AArch64 server).}}

== Lab 3 ==
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### THIS COMMENTED-OUT SECTION DESCRIBES THE 
### CONFIGURATION USED FOR THE WINTER 2014 
### OFFERING OF THE SPO600 COURSE, WHERE THE
### AARCH64 WORK WAS DONE IN EMULATION ALONGSIDE
### THE X86_64 WORK ON THE INTEL HOST "IRELAND".
### IN FALL 2014, AARCH64 HARDWARE WAS AVAILABLE,
### AND IRELAND HAD FAILED, SO WE SWITCHED TO
### THOSE HOSTS.

=== Ireland - Configuration ===

The host ''Ireland'' (ireland.proximity.on.ca) has been set up so that you can use it normally as an x86_64 host, or [[SPO600 aarch64 QEMU on Ireland|use an emulation environment to build and run aarch64 binaries]].

The directory <code>~/arm64/spo600/examples</code>, which is also accessible as <code>~/spo600-examples</code>, contains these files:

 ── hello                         # 'hello world' example programs
    ├── assembler
    │   ├── aarch64               # aarch64 assembler version
    │   │   ├── hello.s
    │   │   └── Makefile
    │   └── x86_64                # x86_64 assembler versions
    │       ├── hello-gas.s       # 64-bit instructions with AT&T/gnu assembler syntax (called 'gas', /usr/bin/as)
    │       ├── hello-nasm.s      # 32-bit instructions with Intel/nasm assembler syntax (/usr/bin/nasm)
    │       └── Makefile
    └── c
        ├── hello2.c              # C version using the write() syscall wrapper
        ├── hello.c               # C version using printf()
        └── Makefile

Throughout this lab, take advantage of ''[[make and Makefiles|make]]'' whenever possible.

-->
=== Code Examples ===

The code examples for this lab are available at either of these links:
* Outside Seneca: http://england.cdot.systems/spo600/spo600-assembler-lab-examples.tgz
* Inside Seneca: http://england.internal.cdot.systems/spo600/spo600-assembler-lab-examples.tgz

Please download this archive to your accounts on the x86_64 and AArch64 systems (<code>wget</code> is a good way to do this), and unpack the archive on both systems.

Unpacking the archive in your home directory will produce the following directory structure:

 spo600
 `-- examples
     `-- hello                     # "hello world" example programs
         |-- assembler
         |   |-- aarch64           # aarch64 gas assembly language version
         |   |   |-- hello.s
         |   |   `-- Makefile
         |   `-- x86_64            # x86_64 assembly language versions
         |       |-- hello-gas.s   # ... gas syntax
         |       |-- hello-nasm.s  # ... nasm syntax
         |       `-- Makefile
         `-- c                     # Portable C versions
             |-- hello2.c          # syscall wrapper version
             |-- hello.c           # printf version
             `-- Makefile

Throughout this lab, take advantage of ''[[make and Makefiles|make]]'' whenever possible.

=== References ===
* [[Assembler Basics]]
* [[x86_64 Register and Instruction Quick Start]]
* [[aarch64 Register and Instruction Quick Start]]

=== Group Lab Tasks ===

1. Build and run the two C versions of the program for x86_64. Take a look at the differences in the code.

2. Use the <code>objdump -d</code> command to dump the object code (machine code) for the two binaries. Find the <code><nowiki><main></nowiki></code> section and make sure you understand the code.

3. Review, build, and run the x86_64 assembly language programs. Make sure you understand the code.

4. Build and run the two C versions of the program for aarch64. Verify that you can disassemble the object code in the ELF binary using <code>objdump -d</code>

5. Review, build, and run the aarch64 assembly language programs. Make sure you understand the code.

6. Here is a basic loop in x86_64 assembler - this loops from 0 to 9, using r15 as the index (loop control) counter:

 .text
 .globl    _start
 
 start = 0                       /* starting value for the loop index; note that this is a symbol (constant), not a variable */
 max = 10                        /* loop exits when the index hits this number (loop condition is i<max) */
 
 _start:
     mov     $start,%r15         /* loop index */
 
 loop:
     /* ... body of the loop ... do something useful here ... */
 
     inc     %r15                /* increment index */
     cmp     $max,%r15           /* see if we're done */
     jne     loop                /* loop if we're not */
 
     mov     $0,%rdi             /* exit status */
     mov     $60,%rax            /* syscall sys_exit */
     syscall

Extend this code, combining it with code from the "Hello World" example, so that it prints each digit from 0 to 9 like this:

 Loop: 0
 Loop: 1
 Loop: 2
 Loop: 3
 Loop: 4
 Loop: 5
 Loop: 6
 Loop: 7
 Loop: 8
 Loop: 9

{{Admon/tip|Character conversion|In order to print the loop index value, you will need to convert from an integer to digit character. In ASCII/ISO-8859-1/Unicode UTF-8, the digit characters are in the range 48-57 (0x30-0x39). You will also need to assemble the message to be printed for each line - you can do this by writing the digit into the message buffer before outputting it to stdout, which is probably the best approach, or you can perform a sequence of writes for the thee parts of the message ('Loop: ', number, '\n'). You may want to refer to the manpage for <code>ascii</code>.}}

7. Repeat step 6 for aarch64.

8. Extend the code to loop from 00-30, printing each value as a 2-digit decimal number.

{{Admon/tip|2-Digit Conversion|You will need to take the loop index and convert it to a 2-digit decimal number by dividing by 10. To do this, use the <code>div</code> instruction, which takes the dividend from rax and the divisor from register supplied as an argument. The quotient will be placed in rax and the remainder will be placed in rdx.}}

9. Repeat step 8 for aarch64.

=== Deliverables ===

1. Complete the group lab section, above.

2. Extend the assembler programs (both x86_64 and aarch64) to suppress the high digit when it is 0. In other words, the printed values should progress from 0-30 instead of from 00-30. It is OK to output a space in place of the suppressed digit (this will cause the numbers to be aligned vertically in the output).

3. Blog about the programs you've written. Describe the experience of writing and debugging in assembler, as compared to writing in other languages. Contrast x86_64 and aarch64 assembler, your experience with each, and your opinions of each. Include links to the source code for both of your assembler programs.

=== Optional Challenge ===

Write a program in aarch64 assembly language to print the times tables from 1-12 ("1 x 1 = 1" through "12 x 12 = 144"). Add a spacer between each table, and use a function/subroutine to format the numbers with leading-zero suppression.

The output could look something like this:

  1 x  1 =   1
  2 x  1 =   2
  3 x  1 =   3
  4 x  1 =   4
  5 x  1 =   5
  6 x  1 =   6
  7 x  1 =   7
  8 x  1 =   8
  9 x  1 =   9
 10 x  1 =  10
 11 x  1 =  11
 12 x  1 =  12
 -------------
  1 x  2 =   2
  2 x  2 =   4
  3 x  2 =   6
  4 x  2 =   8
  5 x  2 =  10
 
  ''' ''...lines snipped for space...'' '''
 
 11 x 12 = 132
 -------------
  1 x 12 =  12
  2 x 12 =  24
  3 x 12 =  36
  4 x 12 =  48
  5 x 12 =  60
  6 x 12 =  72
  7 x 12 =  84
  8 x 12 =  96
  9 x 12 = 108
 10 x 12 = 120
 11 x 12 = 132
 12 x 12 = 144