Logical shifts are operations in which the bits of a register or memory location
are moved to the right or left by a certain number or a value in the CL
register. They are also a very quick way to multiply or divide by 2 or powers of
2 as it involves just a shift of bits. There are 4 shift bit instructions, 4
rotate bit instructions and 2 double precision shift bit instructions for
general purpose registers.
Below is a code snippet that prints a list of prime numbers, one on each line,
based on a limit entered by the user. It uses both while loops and conditional
branch if - else statements. We shall convert this to an assembly program to
demonstrate implementation of these control flow structures in x86-64 assembly.
In the Hello World sample
program we had used the instructions REPNZ and SCASB to calculate the length of the string being printed at runtime. In this program
we use NASM’sequ directive to calculate the length during assembly time as
opposed to at runtime. The variable promptlen gives an example.
Here are some print functions for strings, integers and newline characters.
There is also a function for reading an integer. All the code is in
The macros prologue and epilogue, are used to save space and avoid repetitiveness.
NOTE: Remember that the registers RBP, RBX and R12-R15 need to be saved across function calls.
Below is a program that prints "Hello World!" on screen followed by a newline
character. In the data section we first store the string "Hello World!",
followed by the newline character which has an ASCII value of 10 and the NULL
character or the value 0. The NULL character is used here because of the way we calculate the
string length. There are other ways to calculate the string length as well, by
using NASM’s directives like equ, but we shall use that in another sample program.