This page introduces labels and two types of sections that are common in a RISC-V assembly program.
The two types of sections are the data section and the text section. The data section contains static data allocations while the text section contains assembly instructions. An assembler is a program that converts the text of an assembly program into a binary representation of the program. A directive is used to tell the assembler where each section starts. They direct the assembler to do a specific task or behave in a certain way. A directive always starts with a period.
The directive to start the data section is as follows:
.data
Typically, a section directive appear unindented and alone on the line so that they are easy to visualize. The directive to start the text section is:
.text
There is no need to mark where a section ends. It ends implicitly at the start of another section or at the end of the file. Typically the data section appeard before the text section in an assembly program. However, this order is not a requirement. The overall file structured is as follows:
.data
# Allocate static data
.text
# Assembly code
The # character begins a comment. For more directives see the Data Section Directives.
Labels are names used to identify sections of memory. These names can be used by the programmer to refer to memory locations abstractly. The assembler will translate them into appropriate addresses in the binary representation of the program. A label that appears in the data section, can be thought of as a named variable that you can read from and write to. When used for code, a label can be a procedure name or the target of a jump or a branch instruction. Consider the code snippet below:
.data
exampleLabel1:
.space 1
The label exampleLable1 appears in the data section of the program. A labels
is a string composed of letters, numbers, and underscores. The string is followed
by a colon. A label must start with a letter or an underscore. It cannot start with
a number. A label can be indented but in most styles for assembly programming, labels
are not indented. The label exampleLable1 above will be used in the program to
access the single byte that is allocated by the directive .space 1. This directive
specifies that the assembler must reserve space for one byte at that location in memory.
Whenever the assembler encounters the label exampleLable1 it will replace it with
the address of the byte allocated (see Data Section Directives) .
Labels do not need to be alone on a line. The following line of code is also acceptable:
exampleLabel2: .space 1
When a label appear in the text section, a label can be the destination of a jump or
branch instruction, or it can be the name of a procedure --- the words routine,
subroutine, function, and procedure mean the same thing in assembly programming.
The solution of a programming assignment often consists on writing the assembly code
for a function that executes a specified task. In most programming assignments a main
function is provided and that main function will call the function specified in the
assignment. For example, if an assignment specifies that a function foo that returns
the value zero should be written, the text section of the solution to this assignment will
start with the label foo:
foo:
add a0, zero, zero # t0 <- 0
jalr zero, ra, 0 # Return