Printing a variable from .data

A review of printing with printf

Printing in Assembly always follows the same four steps.

The printing process can be summarized into the following steps:

1. Define the string or format string.
2. Load the address of the string or format string into x0argument and return register (x0-x7): carries the first eight arguments and the return value..
3. Place the corresponding value to be printed into the appropriate argument registers (if any).
4. Call printf.

These steps remain the same when printing variables stored in the .data section. However, the main difference is found in step 3.

In previous lessons, the values being printed were provided directly as immediate values and moved into the appropriate argument registers. For example:

mov x1, 67

Here the value comes out of a .data variable instead of an immediate, but it still ends up in the same argument register.

Loading values from .data

Suppose we have an integer value of 85 stored in a variable named importantNumber within the .data section, as shown below:

To load the value stored in the importantNumber variable defined in the .data section, we must first retrieve the address of the variable. Once the address has been obtained, we can use it to access and load the value stored at that memory location. The following code demonstrates this process:

The ldrLoad from memory. Picks 32-vs-64 bit based on Wt/Xt. instruction is used to retrieve both the address of the variable and the value stored at that address. In this process, two ldrLoad from memory. Picks 32-vs-64 bit based on Wt/Xt. instructions are required.

The first ldrLoad from memory. Picks 32-vs-64 bit based on Wt/Xt. instruction retrieves the address of the variable and stores it in register x21callee-saved register (x19-x28): a routine must restore it before it returns.. The second ldrLoad from memory. Picks 32-vs-64 bit based on Wt/Xt. instruction uses this address to access the value stored in memory and loads the retrieved value into register w1argument and return register (x0-x7): carries the first eight arguments and the return value.. A .word is 32 bits, so the load uses the 32-bit w form of the register; an x1argument and return register (x0-x7): carries the first eight arguments and the return value. load would drag in the 4 bytes stored after the variable too. The register x0argument and return register (x0-x7): carries the first eight arguments and the return value. is preserved for the address of the format string, which is passed to printf as the first argument.

Printing the variable

Once the variable value and format string have been loaded into the appropriate argument registers, the printf function can be called to display the formatted output. The following code demonstrates the complete process: