1. L7 – Assembler Directives
ECE 2560
L7 – Assembler Directives
Department of Electrical and
Computer Engineering
The Ohio State University
ECE Lecture 1

2. Lect 6 – Assembler Directives
What are assembler directives?
The MSP 430 directives to
Specify where user code is
Reserve space for variables
Set the value for constants
Other useful directives
This assembler directives covered in this class are only part of those available.
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3. Assembler Directives What are assembler directives?
How do you set up an area of memory to be data
For temporary data of the program
Set up constants for use in my program
And other possible uses
Assembler Directives are a means by which you can ‘direct’ the assembler to take specific actions.
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4. Where is information on them
Information on the assembler directives can be found in CCS help.
Do a search for assembler directives and the top item is ‘Chapter 5. Assembler Directives.
There are a lot of directives. In class only a small number of them will be addressed.
Section 5.1 is the Directives Summary
Chapter 2 also has information on the assembler directives.
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5. High level directives
The code you develop is divided into sections named
.text – Used for program code (ROM)
.data – Assembles the directives following into the .data section and RAM memory area of the selected MSP430xxxx version.
.intvec – Creates an interrupt vector entry in a named section that points to an interrupt routine name. (located in ROM) More on this later.
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6. When starting a new project
The code template
Note .text section
Code goes here
You need to terminate execution of your code
loop JMP loop
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7. Where to put data?
Use: To reserve space for data variable used in your program.
The data registers are one place to use for the data variables in a program.
Limited to 12 specific locations R5-15.
Program will often have need of more than 12 variables.
The first program could have use mostly register but used memory instead.
Where to store them? IN MEMORY
There needs to be a means to reference them, i.e., they need to be named.
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8. Storing variables in memory
First need a section for variables
.data
Examples of this were shown during the demo.
For example
.word reserves space and initializes the value use:
label .word 0xFFFF
label2 .word 0xAAAA,0x1111,0x2222
.byte reserves space for byte values - use:
label3 .byte 0x11
label4 .byte 0x40
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9. Declare and initialize values
.bits – initializes one or more successive bits (use of one bit will use the entire byte)
.char declares and initialized one or more successive bytes to be the ASCII values of the character.
lbl char 8, “def”
Will place the values 8h in memory, then the ascii for d (64), ascii for e (65), ascii for f (66)
.string – initializes one of more text strings
Mlbl .string “now is the time”
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10. More value types .int – initializes one or more 16-bit integers
.long – initializes one or more 32-bit integers
.float – initializes one or more floating point numbers – IEEE format – 32 bits
.uint – initializes one or more unsigned integers
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11. Just want to reserve space
To reserve space in RAM and attach a label to it so it can be referenced
.bes – Used for uninitialized objects (global data – variables)
label .bes size label points to the end of the reserved space
.space – Used for uninitialized objects
splbl .space size splbl points to the start of the reserved space
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12. For further reference
For a further explanation of the assembler directive you can look in section 5.12, Directives Reference, in the MSP430 Assembly Language Tool User’s Guide (available in Code Composer help).
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13. Summary - Assignment
These are the most relevant directives for this class.
No new assignment.
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