Transcript Building a simple loop using Blackfin assembly code

Building a simple loop using
Blackfin assembly code
M. Smith,
Electrical and Computer Engineering,
University of Calgary, Canada
Tackled on Tutorial
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Bring Blackfin Instruction Reference Manual
to the Tutorial
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Okay to work on exercises with your lab. partner
Determine the differences / advantages and
disadvantages between
for-loops, while-loops, do-while-loops and
do-while-loops with initial tests
Demonstrate ability to turn functioning “C++”
into Blackfin assembly code
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Stub for
void SimpleLoopASM (ushort)
#include <defBF533.h>
#include <macros.h>
.section program;
.global _SimpleLoopASM;
.align 2;
#define SimpleLoopASMSTACK 16
_SimpleLoopASM:
// void SimpleLoopASMTime (unsigned short int timeToUse) {
LINK SimpleLoopASMSTACK;
// Code goes here and replaces next line
R0 = 0;
P0 = [FP + 4 ];
UNLINK;
JUMP (P0);
// }
_SimpleLoopASM.end:
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C++ version of code
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Could be constructed using “for-loop”
void SimpleLoop(unsigned short int timeToUse) {
unsigned short int counter = 0;
for (counter = 0; count <= timeToUse; count++) {
counter = counter; // Waste time
}
// Spin the wheels
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Translation problems with
“for-loop” into assembly code
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Most processors don’t any capability to directly
perform “for-loops” in assembly code.
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Blackfin has “limited” capability
MIPS has ?
68000 has ?
Time spent in function depends on capabilities of
compiler and processor
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An optimizing compiler may recognize that “nothing useful”
is happening in the loop and remove it from the function
Loop speed depends on processor speed – improve the
processor means code speed is faster
Original “Invaders” game on Atari processor used this as a
“feature” and not a bug.
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C++ version of code
Could be constructed using “while” and “do while” constructs
WHILE
unsigned short int counter = 0;
while (counter <= timeToUse) {
counter++;
}
NOTES ON ISSUES WITH
WHILE AND DO- WHILE
CONSTRUCTS
DO_WHILE
unsigned short int counter = 0;
do {
counter++;
} while (counter <= timeToUse) {
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Can now develop / test a
“C++” prototype function
WHILE
unsigned short int counter = 0;
while (counter <= timeToUse) {
counter++;
}
PREPARE FOR ASSEMBLY CODE TRANSLATION
unsigned short int counter = 0;
WHILE: IF (counter <= timeToUse)
then JUMP to ENDWHILE label
ELSE {
counter++;
JUMP to WHILE label
}
ENDWHILE:
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Knowledge needed to
continue
unsigned short int counter = 0;
WHILE: IF (counter <= timeToUse)
then JUMP to ENDWHILE label
ELSE {
counter++;
JUMP to WHILE label
}
ENDWHILE:
What register is suitable to store the counter value?
How is the parameter timeToUse being passed to the function?
How do you do a conditional jump?
How do you do a test such as counter < timeToUse?
What is the difference between a loop using a signed value timeToUse
and one using an unsigned value timeToUse?
What is the difference between a loop using a short int value timeToUse
and one using a long int value timeToUse?
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How do you do a
conditional jump on a Blackfin?
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Key reference material
Blackfin Instruction Manual
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Chapter 2 Program Flow Control
Instruction Summary
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• “Jump” on page 2-2
• “IF CC JUMP” on page 2-5
• “Call” on page 2-8
• “RTS, RTI, RTX, RTN, RTE (Return)”
on page 2-10
• “LSETUP, LOOP” on page 2-13
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How do you do a
conditional jump on a Blackfin?
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Add the answer
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The condition code register
CC
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This is the Blackfin Boolean condition
code or flag register
Can take the value TRUE = 1
Can take the value FALSE = 0
Legal conditional MOVE instructions?
IF NOT LEGAL WHY NOT?
 IF CC R4 = R5; IF !CC R6 = R7;
 IF CC P0 = R5;
IF !CC P2 = P7;
 IF CC R0 = R7.L; IF !CC R0.L = R4.L
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The condition code register
CC
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Reference Blackfin Instruction Manual
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Chapter 4, MOVE instruction
Legal conditional MOVE instructions?
IF NOT LEGAL WHY NOT?
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IF CC R4 = R5; IF !CC R6 = R7;
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IF CC P0 = R5;
IF !CC P2 = P7;
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IF CC R0 = R7.L; IF !CC R0.L = R4.L
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ANSWER HERE
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How to we set the
CC register?
Reference Blackfin Instruction Manual
Chapter 6, Condition Code Bit Management
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CC = Dreg == Dreg ; /* equal, register, signed (a) */
CC = Dreg < Dreg ; /* less than, register, signed (a) */
CC = Dreg <= Dreg ; /* less than or equal, register, signed (a) */
CC = Dreg == imm3 ; /* equal, immediate, signed (a) */
CC = Dreg < imm3 ; /* less than, immediate, signed (a) */
CC = Dreg <= imm3 ; /* less than or equal, immediate, signed (a) */
CC = Dreg < Dreg (IU) ; /* less than, register, unsigned (a) */
CC = Dreg <= Dreg (IU) ; /* less than or equal, register, unsigned (a)
CC = Dreg < uimm3 (IU) ; /* less than, immediate, unsigned (a) */
CC = Dreg <= uimm3 (IU) ; /* less than or equal, immediate unsigned
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Important to know what you
CAN’T DO
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YOU CAN DO
YOU CAN’T DO
CC = R1 == R2
CC = (R1 == R2);
YOU CAN DO
YOU CAN’T DO
BUT YOU CAN DO
CC = R1 < 3;
CC = R1 < 7;
CC = R1 < 7 (IU);
YOU CAN DO
YOU CAN’T DO
CC = R1 < -3;
CC = R1 < -3 (IU);
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LEGAL OR NOT?
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CC = R1.L < 2;
CC = R1.L < 9;
CC = R1.L < R2.L;
CC = P3 <= P4;
CC = P3 < 4;
R3 = CC;
R4 = R5 – R6; then CC = AZ;
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Now you have enough information to
code “while” and “do-while” functions
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While loop function in Blackfin
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Now you have enough information to
code “while” and “do-while” functions
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Do-While loop function in Blackfin
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Questions to answer
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Number of instructions in do-while loop
function
Number of instructions in while loop function
Number of jump operations (each time
round the loop) with do-while loop function
Number of jump operations (each time
round the loop) with while loop function
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Are there advantages for a Dowhile loop with an initial test?
WHILE
unsigned short int counter = 0;
while (counter <= timeToUse) {
counter++;
}
DO_WHILE
unsigned short int counter = 0;
do {
counter++;
} while (counter <= timeToUse)
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DO_WHILE WITH INITIAL TEST
unsigned short int counter = 0;
if (counter > timeToUse) {
do {
counter++;
} while (counter <= timeToUse)
}
Building a simple loop using Blackfin assembly code
Copyright M. Smith, University of Calgary, Canada
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Code the do-while loop with
initial test
DO_WHILE WITH INITIAL TEST
unsigned short int counter = 0;
if (counter > timeToUse) {
do {
counter++;
} while (counter <= timeToUse)
}
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Major problem
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A major problem with any form of loop
is the “one-off” problem
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You go round the loop one time too many
You go round the loop one time too few
Do any of the code examples in this
lecture suffer from this problem?
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Tackled Today
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Differences in behaviour between
for-loops, while-loops, do-while-loops,
do-while loops with initial test
Conditional JUMP and
Conditional MOVE instructions
Setting the CC condition code register
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What you would like to do, and can
What you would like to do, but can’t
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Information taken from Analog Devices On-line
Manuals with permission
http://www.analog.com/processors/resources/technicalLibrary/manuals/

Information furnished by Analog Devices is believed to
be accurate and reliable. However, Analog Devices
assumes no responsibility for its use or for any
infringement of any patent other rights of any third party
which may result from its use. No license is granted by
implication or otherwise under any patent or patent right
of Analog Devices. Copyright  Analog Devices, Inc. All
rights reserved.
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