Program 5: Hello World Revisited
Including Other Files, Pt. 1 #
MACRO directive #
Putting everything in one file is great keeping code together, but absolutely awful for more than trivial programs as the number of lines of code grows rapidly. There are two ways you can include code in from other files and this is the first way. Like a keyboard macro, the MACRO directive puts an exact copy of the code at the point of inclusion.
@ the syntax is .macro followed by the name of the macro followed by a
@ comma seperated list of arguments expected.
.macro macroname arg1, arg2, arg3
@ arguments are inserted in the code with a backslash
mov r0, \arg1 @ loads the immediate into r0
@ macros are ended by the .endm directive
.endm
Local labels #
Since the macro includes the code verbatim, you need to consider what would not work as if everything were in one file. One such instance would be labels as they need to be unique within the file. We can accomplish this by using local labels. Local labels are a number from 0-99 followed by a colon. You can then append either a -f or a -b to look for that number forward or backwards.
1:
add r0, #1 @ add 1 to r0
cmp r0, #5 @ compare 0 to 5
beq 2f @ go on if equal
b 1b
2:
@ Code continues....
INCLUDE directive #
The last part of this is to actually include the file that has the macro if we put it in an external file. We do this with the include directive. The include directive simply includes the file, in the exact space, as if it were the same file.
Null terminated strings #
When you use a macro, you start getting into an area where you need to trust the person using your code to provide things such as output memory space. So we need to create a macro that can be used in any program, even if we don’t know the length. One way programs do this is by null terminated data. It’s such a great idea, there’s a macro for that too! The two following data items will load the same data into memory.
.data
explictnull: .ascii "String with null\0"
datanull: .asciz "String with null"