Transcript Building a simple loop using Blackfin assembly code

Building a simple loop using
Blackfin assembly code
If you can handle the while-loop correctly, then
most of the laboratories are easy to complete
M. Smith,
Electrical and Computer Engineering,
University of Calgary, Canada
Tackled today
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Bring Blackfin Instruction Reference Manual
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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C++ version of code -- “for-loop”
int SimpleForLoopCPP(void) {
int counter = 0;
int sum = 0;
for (counter = 0; count <= 15; count++) {
sum += counter;
} – Most processors have to “fake” for loops
return sum;
}
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Standard assembly stub code for
int SimpleForLoopASM (void)
#include <defBF533.h>
#include <macros.h>
.section program;
.global _SimpleForLoopASM;
.align 4;
#define SimpleForLoopASMSTACK 16
_SimpleForLoopASM:
// void SimpleForLoopASM (void) {
LINK SimpleForLoopASMSTACK;
// Code for loop goes here
P0 = [FP + 4 ];
UNLINK;
JUMP (P0);
// }
_SimpleForLoopASM.end:
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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 loop 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
while-loop and do-while loop
Could be constructed using “while” and “do while” constructs
WHILE
int counter = 0;
while (counter <= 15) {
sum += counter;
counter++;
}
NOTES ON ISSUES WITH
WHILE AND DO- WHILE
CONSTRUCTS
DO_WHILE
int counter = 0;
do {
sum += counter;
counter++;
} while (counter <= 15) ;
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Can now develop / test a
“C++” prototype function
WHILE
int counter = 0;
while (counter <= 15) {
sum += counter;
counter++;
}
CHANGE CODE FORMAT TO
PREPARE FOR ASSEMBLY CODE TRANSLATION – 1 ACTION PER LINE
int counter = 0;
WHILE: IF (counter > 15)
then JUMP to ENDWHILE label
ELSE {
sum += counter;
counter++;
JUMP to WHILE label 
}
ENDWHILE:
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THIS LINE IS OFTEN MISSED
Building a simple loop using Blackfin assembly code
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Knowledge we need to have in
order to be able to continue
int counter = 0;
WHILE: IF (counter > 15)
then JUMP to ENDWHILE label
ELSE {
sum += counter;
counter++;
JUMP to WHILE label
}
ENDWHILE:
What register is suitable to store the counter variable value?
What register is suitable to store the sum variable value?
How do you do a conditional jump?
How do you do a test such as counter > 2?
How do you do a test such as counter > 15? -- There IS a difference
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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 7 Program Flow Control
Instruction Summary
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• “Jump” on page 7-2
• “IF CC JUMP” on page 7-5
• “Call” on page 7-8
• “RTS, RTI, RTX, RTN, RTE (Return)”
on page 7-10
• “LSETUP, LOOP” on page 7-13
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How do you do a conditional jump
to instruction label END_WHILE:
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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
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Can take the value TRUE = 1
Can take the value FALSE = 0
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Can be set in a number of different
ways
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The condition code register CC used to
conditionally move register values
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Legal conditional MOVE instructions?
IF NOT LEGAL WHY NOT? CONDITIONAL MOVE
CHECK THEM OUT BY LOOKING IN CHAPTER 9-8!!
 IF CC R4 = R5; // If CC TRUE then set R4 = R5
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Meaning MOVE the value in R5 into R4 (leaves R5 unchanged)
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IF !CC R6 = R7; // If CC FALSE then set R4 = R5
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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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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 when setting CC!!
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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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WHAT ARE WE TRYING TO DO?
IS IT LEGAL SYNTAX OR NOT?
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CC = R1 < 2; // Attempting to test 32-bit DATA register
CC = R1 < 9 ; // Attempting to test 32-bit register
CC = R1.L < 2 ; // Attempting to test 16-bit register
CC = R1.L < 9 ; // Attempting to test 16-bit register
CC = P3 <= P4;
CC = P3 < 4;
// Attempting to test 32-bit POINTER register
// Attempting to test 32-bit POINTER register
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R3 = 1;
CC = R3;
// Putting TRUE value into R3?
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R4 = R5 – R6; // Math operations – sets the Arithmetic Zero flag
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CC = AZ;
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Now you have enough information to
code “while” operations in Blackfin
Translate line-by-line
int counter = 0;
WHILE: IF (counter <= 15)
then JUMP to ENDWHILE label
ELSE {
sum += count;
counter++;
JUMP to WHILE label
}
ENDWHILE:
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Code the do-while loop
DO_WHILE
int counter = 0;
do {
sum += counter;
counter++;
} while (counter <= 15);
}
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Are there advantages for a Dowhile loop with an initial test?
WHILE
int counter = 0;
while (counter <= 15) {
sum += counter;
counter++;
}
JUMPS NEEDED IN RED
DO_WHILE
int counter = 0;
do {
sum += counter;
counter++;
} while (counter <= 15)
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FASTEST LOOP
DO_WHILE WITH INITIAL TEST
int counter = 0;
// Unknown value in “avalue”
if (counter > avalue) {
do {
sum += counter;
counter++;
} while (counter <= avalue)
}
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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Questions to answer at a later time
Speed -- timing issues
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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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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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Blackfin “real” for-loops
Needed in Assignment 1
int counter = 0;
int sum = 0;
for (counter = 0;
count <= 15;
count++) {
sum += counter;
}
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Chapter 7
Form 1 – big loops
LOOP loop_name loop_counter
LOOP_BEGIN loop_name
LOOP_END loop_name
Form 2 – small loops
LSETUP (Begin_Loop, End_Loop) loop_counter
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Blackfin “real” for-loops
Needed in Assignment 1
int counter = 0;
int sum = 0;
for (counter = 0;
count <= 15;
count++) {
sum += counter;
}
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Chapter 7
Form 1 – big loops
LOOP loop_name loop_counter
LOOP_BEGIN loop_name
LOOP_END loop_name
Form 2 – small loops
LSETUP (Begin_Loop, End_Loop) loop_counter
Special loop control registers
Loop_Top
Loop_Bottom LB1 and LB0 (address)
Loop_Count LC1 and LC0 (number)
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LT1 and LT0 (address)
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Code the “big loop” format
int counter = 0;
int sum = 0;
for (counter = 0;
count <= 15;
count++) {
sum += counter;
}
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Form 1 – big loops
LOOP loop_name loop_counter
LOOP_BEGIN loop_name
LOOP_END loop_name
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Code the “small loop” format
int counter = 0;
int sum = 0;
for (counter = 0;
count <= 15;
count++) {
sum += counter;
}
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Form 2 – small loops
LSETUP (Begin_Loop, End_Loop) loop_counter
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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/
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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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