I have found the assembly language intriguing and challenging on both the group task and later on during the individual work process I have gone through.
After reviewing the code piece that was given to us (the group) and printed "Loop" The first task was to modify our loop program so that it counts from 0-9.
This was done by setting a conversion of an integer to digital character in ASCII/ISO-8859-1/Unicode UTF-8 which would be 48-57 (0x30-0x39).
After initialising the loop index value to 0 and moving it to the registry, I have added the current loop value to %r8 (which is register number 8 in the 64-bit mode.
Next step, would be to set the current number to the string, to a variable called pos.
The print function is similar to the one shown in the Assembler Basics tutorial, as it fulfills the needs of the program.
Code that was developed in class printed the result:
In order to get the code to print two digits value and print a loop that counts from 0-30, we would initialise another digit and divide the number by 10 so we can present the remainder as the second digit. Then we can assign the quotient and remainder to the register r8 and r9.Loop: 0 Loop: 1 Loop: 2 Loop: 3 Loop: 4 Loop: 5 Loop: 6 Loop: 7 Loop: 8 Loop: 9
Code developed:
.text
.global _start
sout = 1
start = 0 /* starting value for the loop index; note that this is a symbol (constant), not a variable */
max = 31 /* loop exits when the index hits this number (loop condition is i<max) */
_start:
mov $start,%r15 /* loop index */
loop:
/* Set to 0 */
mov $48, %r8
mov $48, %r9
/* Calculation */
mov $0,%rdx
mov $10,%r10
mov %r15,%rax
div %r10 /* division */
add %rax,%r8
add %rdx,%r9
/* add the current number */
mov %r8b, pos
mov %r9b, posB
cmp $0x30, %r15
jmp continue
/* print (taken from example) */
mov $len,%rdx /* message length */
mov $msg,%rsi /* message location */
mov $sout,%rdi /* file descriptor stdout */
mov $1,%rax /* syscall sys_write */
syscall
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
.data
msg: .ascii "Loop: \n"
.set len , . - msg
/* set position of number right after the msg */
.set pos , msg + 6
.set posB , msg + 7
Would print the following result:
Loop: 0
Loop: 1
Loop: 2
Loop: 3
Loop: 4
Loop: 5
Loop: 6
Loop: 7
Loop: 8
Loop: 9
Loop: 10
Loop: 11
Loop: 12
Loop: 13
Loop: 14
Loop: 15
Loop: 16
Loop: 17
Loop: 18
Loop: 19
Loop: 20
Loop: 21
Loop: 22
Loop: 23
Loop: 24
Loop: 25
Loop: 26
Loop: 27
Loop: 28
Loop: 29
Loop: 30
Aarch64:
The aarch64 version is quite similar to the x86_64 version, logic-wise, therefore the same processes can be applied to it as well. However, the instructions, registers and orders were different. Goes to show how meticulous is the process of debugging Assembly language.
I have found it more difficult to debug on Aarch64.
The task we were assigned to do was to create a loop that goes from 0 to 30.
The code that was developed:
.text
.global _start
sout = 1
start = 0
max = 31
_start:
mov x4, start
loop:
/* set */
mov w8, 48
mov w9, 48
/* calculations */
mov w2, 10
udiv w1, w4, w2
msub w5, w1, w2, w4
add w8, w8, w1
add w9, w9, w5
adr x1, msg
strb w9, [x1, 7]
cmp w8, 0x30
beq continue
strb w8, [x1, 6]
continue:
/* print */
mov x0, sout
mov x2, len
mov x8, 64
svc 0
add x4, x4, 1
cmp x4, max
b.ne loop
mov x0, 0 /* status -> 0 */
mov x8, 93 /* exit is syscall #93 */
svc 0 /* invoke syscall */
/* print data */
.data
msg: .ascii "Loop: \n"
len= . - msg
Would print the following result:
Loop: 0
Loop: 1
Loop: 2
Loop: 3
Loop: 4
Loop: 5
Loop: 6
Loop: 7
Loop: 8
Loop: 9
Loop: 10
Loop: 11
Loop: 12
Loop: 13
Loop: 14
Loop: 15
Loop: 16
Loop: 17
Loop: 18
Loop: 19
Loop: 20
Loop: 21
Loop: 22
Loop: 23
Loop: 24
Loop: 25
Loop: 26
Loop: 27
Loop: 28
Loop: 29
Loop: 30
In conclusion
I have found Assembly very intriguing, the more you go in depth with it, yet unnecessarily challenging, especially nowadays. The coding on the Aarch64 server was, personally, more difficult to figure out and to solve, due to difficulties I have had with the initial loop.
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