Linux Syscalls: Return values of Registers and Flags

Use gdb script
Write it to a file or add it to the end of this file.: ~/.gdbinit

Code:

define dregs
  printf "\e[1;1H\e[10000C\e[16D\e[30;47mEAX=\e[94;47m%04X%02X%02X\e[2;1H\e[10000C\e[16D\e[30;47mEBX=\e[94;47m%04X%02X%02X\e[3;1H\e[10000C\e[16D\e[30;47mECX=\e[94;47m%04X%02X%02X\e[4;1H\e[10000C\e[16D\e[30;47mEDX=\e[94;47m%04X%02X%02X\e[5;1H\e[10000C\e[16D\e[30;47mESP=\e[94;47m%04X%04X\e[6;1H\e[10000C\e[16D\e[30;47mEBP=\e[94;47m%04X%04X\e[7;1H\e[10000C\e[16D\e[30;47mESI=\e[94;47m%04X%04X\e[8;1H\e[10000C\e[16D\e[30;47mEDI=\e[94;47m%04X%04X\e[9;1H\e[10000C\e[16D\e[30;47mEIP=\e[94;47m%08X\e[9;1H", (unsigned short)((unsigned)$eax >> 16), (unsigned char)$ah, (unsigned char)$al, (unsigned short)((unsigned)$ebx >> 16), (unsigned char)$bh, (unsigned char)$bl, (unsigned short)((unsigned)$ecx >> 16), (unsigned char)$ch, (unsigned char)$cl, (unsigned short)((unsigned)$edx >> 16), (unsigned char)$dh, (unsigned char)$dl, (unsigned short)((unsigned)$esp >> 16), (unsigned short)$sp, (unsigned short)((unsigned)$ebp >> 16), (unsigned short)$bp, (unsigned short)((unsigned)$esi >> 16), (unsigned short)$si, (unsigned short)((unsigned)$edi >> 16), (unsigned short)$di, (unsigned)$pc
end

define showflags
  printf "%08X\e[1B\e[9D%X%X%X%X%X%X%X%X%X\e[1B\e[9DDTIOSZAPC", $eflags, (unsigned)(((unsigned)$eflags >> 10) & 1), (unsigned)(((unsigned)$eflags >> 8) & 1), (unsigned)(((unsigned)$eflags >> 9) & 1), (unsigned)(((unsigned)$eflags >> 11) & 1), (unsigned)(((unsigned)$eflags >> 7) & 1), (unsigned)(((unsigned)$eflags >> 6) & 1), (unsigned)(((unsigned)$eflags >> 4) & 1), (unsigned)(((unsigned)$eflags >> 2) & 1), (unsigned)(((unsigned)$eflags >> 0) & 1)
end

You can use this command sequence to run in the gdb. Just change the ./fasm in two places to the name of your executable file.

Code:

$ cp ~/.gdbinit ~/.gdbinit.old && readelf -h ./fasm | grep 'Address' | awk '{print "break *"$NF}' >> ~/.gdbinit && gdb -q ./fasm && mv ~/.gdbinit.old ~/.gdbinit

If gdb was launched using the command I suggested, then it is enough to execute the run or r command and execution will stop at the entry point to the program.
In gdb, you will have two new user commands in addition to the existing (inconvenient) info registers.
dregs for displaying 32-bit registers with formatting on the screen
showflags for displaying the flags register as a set of signed bits

ADD: You can clearly see how the registers change after the int 0x80 command.

(only the eax changes)