blackfin loop asm
TRANSCRIPT
Building a simple loop using Blackfin assembly code
M. Smith,
Electrical and Computer Engineering,
University of Calgary, Canada
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Tackled on Tutorial
Bring Blackfin Instruction Reference Manual to the Tutorial 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
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
Most processors don’t any capability to directly perform “for-loops” in assembly code. Blackfin has “limited” capability MIPS has ? 68000 has ?
Time spent in function depends on capabilities of compiler and processor 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++; }
DO_WHILE unsigned short int counter = 0; do {
counter++; } while (counter <= timeToUse) {
NOTES ON ISSUES WITH WHILE AND DO- WHILE CONSTRUCTS
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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?
Key reference materialBlackfin Instruction Manual
Chapter 2 Program Flow Control Instruction Summary
• “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?
Add the answer
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The condition code register CC 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 Reference Blackfin Instruction Manual
Chapter 4, MOVE instruction 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
ANSWER HERE
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How to we set the CC register?Reference Blackfin Instruction Manual
Chapter 6, Condition Code Bit Management 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
YOU CAN DO CC = R1 == R2 YOU CAN’T DO CC = (R1 == R2);
YOU CAN DO CC = R1 < 3; YOU CAN’T DO CC = R1 < 7; BUT YOU CAN DO CC = R1 < 7 (IU);
YOU CAN DO CC = R1 < -3; YOU CAN’T DO CC = R1 < -3 (IU);
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LEGAL OR NOT?
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
While loop function in Blackfin
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Now you have enough information to code “while” and “do-while” functions
Do-While loop function in Blackfin
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Questions to answer
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 Do-while 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)
DO_WHILE WITH INITIAL TEST unsigned short int counter = 0;
if (counter > timeToUse) {do { counter++;
} while (counter <= timeToUse)
}
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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
A major problem with any form of loop is the “one-off” problem 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
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 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.