DM548 Computerarkitektur og systemprogrammering - 2018

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Small examples, links and hints for DM548 Computer Architecture and system programming - 2018

Link to some information of flags when opening files. Jakob's slide should also be updated we needed information:
https://code.woboq.org/userspace/glibc/sysdeps/unix/sysv/linux/bits/fcntl-linux.h.html

C-example for http://www.pythontutor.com/c.html#mode=display

http://www.pythontutor.com/c.html#mode=display

void swap(int* a, int* b) {
  int* temp_1;
  int* temp_2;
  temp_1 = a;
  temp_2 = b;
  a = temp_2;
  b = temp_1;
}

void swap_pointers(int** a, int** b) {
  int* temp_1;
  int* temp_2;
  temp_1 = *a;
  temp_2 = *b;
  *a = temp_2;
  *b = temp_1;
}

int main() {
  int x = 6; 
  int *y;
  int z[10];
  int **a;
  
  y = malloc(sizeof(int));
  *y = 8;
  
  z[0] = 1;
  z[1] = 2;
  z[2] = 3;
  z[3] = 4;
  z[4] = 5;
  
  
  a = malloc(sizeof(int *) * 10);
  a[0] = y;
  z[1] = y;
  
  a[1] = 2;
  
  int* temp;
  temp = malloc(sizeof(int));
  *temp = 42; 
  a[2] = temp;
  free(temp);
  
  a[4] = &z[3];
  a[5] = y;
  
  swap(a[4], a[5] );
  swap_pointers(&a[4], &a[5] );
  
  return 0;
}

Very simple Makefile for the file hello.c:

SOURCES = hello.c
OBJS := hello.o
CFLAGS = -g

all: hello

hello: $(OBJS)
	cc $(OBJS) $(CFLAGS) -o hello

clean:
	rm -f $(OBJS) hello

Example of recursion:

note: The function print_rax is used from another file (provided by Jakob) and is linked by the 'ld' command when linking this file. note: The function is calculating the Fibonacci "(%rax + 1)"-th number. This means if %rax has value 5 we calculate the 6-th Fibonacci number.

.global  _start
_start:

movq $5, %rax  # find the 6th Fibonacci number 
movq $1, %rbx  # Set to 1
movq $0, %rcx  # Set to 0

call print_rax

call FibFunction
movq %rdx, %rax

call print_rax

# Syscall calling sys_exit
movq $60, %rax            # rax: int syscall number
movq $0, %rdi             # rdi: int error code
syscall

FibFunction:
	push %rbp
	movq %rsp, %rbp
	movq $0, %rdx 
	# If needed:
	# subq 
	#pusha # Save all general purpose registers

	# If needed:
	#movq (rbp+16 + 8*0), %rax # first parameter on stack
	#movq [rbp+16 + 8*1], %rbx # second parameter on stack
	#movq [rbp+16 + 8*2], %rcx # third parameter on stack

	addq %rcx, %rdx 
	addq %rbx, %rdx # rdx hold the n-th Fibonacci number
	movq %rbx, %rcx # If we want an itteration more, this is now n-2
	movq %rdx, %rbx # This will be n - 1
	
	subq $1, %rax
	cmpq $0, %rax
	je done

	call FibFunction

	done:

	#popa # Restore all general purpose registers
	pop %rbp
	ret

SSE Instruction example:

.section  .data
.align 16 # add  padding  bytes so the  next  thing is 16-byte  aligned
ss: .single 0.1, 0.2, 0.3, 0.4
ds: .double 1.1, 1.2
.section  .text
.global  _start
_start:
movaps ss , %xmm0
movapd ds , %xmm1

# Syscall calling sys_exit
movq $60, %rax            # rax: int syscall number
movq $0, %rdi             # rdi: int error code
syscall

How to run the file:

Save the file as xmm.s and compile it like this:
as -g -o xmm.o xmm.s
Then link is by
ld -o xmm.bin xmm.o
Now we can run the binary with gdb:
gdb ./xmm.bin
Set a breakpoint:
break _start
Start the program:
run
Run the first instructions to move the numbers into the registers:
nexti
nexti
Now print the content of the xmm0 and xmm1 registers:
p $xmm0
p $xmm1
You should now be able to see that the values we have specified in the file now is in the registers.
Continue to use the nexti command untill you can see that the program exited normally.