Building a simple loop using Blackfin assembly code

Building a simple loop using Blackfin assembly code If you can handle the while-loop correctly in assembly code on any processor, then most of the other code is easy to complete. M. Smith, Electrical and Computer Engineering, University of Calgary, Canada

Tackled today n n n 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 5/22/2021 Building a simple loop using Blackfin assembly code Copyright M. Smith, University of Calgary, Canada 2 / 27

C++ version of code -- “for-loop” int Simple. For. Loop. CPP(void) { int counter = 0; int sum = 0; for (counter = 0; count <= 15; count++) { sum += counter; } – Most processors have to “fake” for-loops return sum; } 5/22/2021 Building a simple loop using Blackfin assembly code Copyright M. Smith, University of Calgary, Canada 3 / 27

Standard assembly stub code for int Simple. For. Loop. ASM (void) #include <def. BF 533. h> #include <macros. h>. section program; . global _Simple. For. Loop. ASM; . align 4; // void Simple. For. Loop. ASM (void) { _Simple. For. Loop. ASM: // Code for loop goes here _Simple. For. Loop. ASM. end: 5/22/2021 RTS; // } A “stub” is NOT designed to “work properly”. It is designed to have just enough information so that you can compile or assembly, build and run the project without crashing the system. If you can’t get the stub to “run”, then the “real” code will not either. Building a simple loop using Blackfin assembly code Copyright M. Smith, University of Calgary, Canada 4 / 27

Translation problems with “for-loop” into assembly code n Most processors don’t any capability to directly perform real “for-loops” in assembly code. ¡ ¡ ¡ n Blackfin has “limited” capability MIPS has ? 68000 has ? Time spent in loop depends on capabilities of compiler and processor ¡ ¡ ¡ 5/22/2021 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. Building a simple loop using Blackfin assembly code Copyright M. Smith, University of Calgary, Canada 5 / 27

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) ; 5/22/2021 Building a simple loop using Blackfin assembly code Copyright M. Smith, University of Calgary, Canada 6 / 27

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 THIS LINE IS OFTEN MISSED } // Since missing that last jump line wastes so much time in the laboratory ENDWHILE: // then missing that last jump line in quizzes and exams is marked hard 5/22/2021 Building a simple loop using Blackfin assembly code Copyright M. Smith, University of Calgary, Canada 7 / 27

Knowledge we need to have in order to be able to continue 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 int counter = 0; WHILE: IF (counter > 15) then JUMP to ENDWHILE label ELSE { sum += counter; counter++; JUMP to WHILE label } ENDWHILE: 5/22/2021 while (counter <= 15) { sum += counter; counter++; } Building a simple loop using Blackfin assembly code Copyright M. Smith, University of Calgary, Canada 8 / 27

How do you do a conditional jump on a Blackfin? n n Key reference material Blackfin Instruction Manual Chapter 7 Program Flow Control ¡ Instruction Summary n n n 5/22/2021 Setting the CC register on page ? ? ? “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 Building a simple loop using Blackfin assembly code Copyright M. Smith, University of Calgary, Canada 9 / 27

The condition code register CC n This is the Blackfin Boolean condition code or flag register n CC can take the value TRUE = 1 CC can take the value FALSE = 0 n n n CC register can be set in a number of different ways Blackfin processor can actually do “IF” instructions in assembly code 5/22/2021 Building a simple loop using Blackfin assembly code Copyright M. Smith, University of Calgary, Canada 10 / 27

The condition code register CC used to conditionally move register values n Are these legal conditional MOVE instructions? IF NOT LEGAL WHY NOT? CONDITIONAL MOVE CHECK THEM OUT BY LOOKING IN CHAPTER 9 -8!! ¡ IF CC R 4 = R 5; // If CC TRUE then set R 4 = R 5 n Meaning MOVE the value in R 5 into R 4 (leaves R 5 unchanged) ¡ IF !CC R 6 = R 7; // If CC FALSE then set R 4 = R 5 ¡ IF CC P 0 = R 5; // Move a value from a data register IF !CC P 2 = P 7; // into a pointer register ¡ ¡ ¡ IF CC R 0 = R 7. L; // Move a 16 bit value into a 32 bit register IF !CC R 0. L = R 4. L // Move a 16 -bit value into a 16 bit register 5/22/2021 Building a simple loop using Blackfin assembly code Copyright M. Smith, University of Calgary, Canada 11 / 27

How to we set the CC register? Reference Blackfin Instruction Manual Chapter 6, Condition Code Bit Management n n n n n 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 == imm 3 ; /* equal, immediate, signed (a) */ CC = Dreg < imm 3 ; /* less than, immediate, signed (a) */ CC = Dreg <= imm 3 ; /* 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 < uimm 3 (IU) ; /* less than, immediate, unsigned (a) */ CC = Dreg <= uimm 3 (IU) ; /* less than or equal, immediate unsigned 5/22/2021 Building a simple loop using Blackfin assembly code Copyright M. Smith, University of Calgary, Canada 12 / 27

How do you do a conditional jump to instruction label END_WHILE: n Add the answer n First set condition code flag (CC) to be true or false 5/22/2021 Building a simple loop using Blackfin assembly code Copyright M. Smith, University of Calgary, Canada 13 / 27

Important to know what you CAN’T DO when setting CC!! n n YOU CAN DO YOU CAN’T DO CC = R 1 == R 2 CC = (R 1 == R 2); YOU CAN DO YOU CAN’T DO CC = R 1 < 3; CC = R 1 < 15; (too big) only 3 -bits available in “this” signed CC instruction – test -4 to +3 n n n BUT YOU CAN DO the two instruction operation (I always do this one) R 2 = 15; CC = R 1 < R 2; YOU CAN DO YOU CAN’T DO CC = R 1 < -3; CC = R 1 < -3 (IU); only 3 -bits available in “this” unsigned CC instruction – test 0 to +7 5/22/2021 Building a simple loop using Blackfin assembly code Copyright M. Smith, University of Calgary, Canada 14 / 27

WHAT ARE WE TRYING TO DO? IS IT LEGAL SYNTAX OR NOT? n n CC = R 1 < 2; CC = R 1 < 9 ; CC = R 1. L < 2 ; CC = R 1. L < 9 ; // Attempting to test 32 -bit DATA register // Attempting to test 32 -bit register // Attempting to test 16 -bit register CC = P 3 <= P 4; CC = P 3 < 0 x 44; // Attempting to test 32 -bit POINTER register n R 3 = 1; CC = R 3; // Putting TRUE value into R 3? n R 4 = R 5 – R 6; // Math operations – sets the Arithmetic Zero flag n n CC = AZ; 5/22/2021 Building a simple loop using Blackfin assembly code Copyright M. Smith, University of Calgary, Canada 15 / 27

Now you have enough information to code “while” operations in Blackfin Translate line-by-line sum_R 0 = 0; counter_R 1 = 0; max_count_R 2 = 15; WHILE: ? ? // Loop body sum_R 0 = sum_R 0 + counter_R 1; ? ? int counter = 0; WHILE: IF (counter <= 15) then JUMP to ENDWHILE label ELSE { sum += counter; counter++; JUMP to WHILE label } ENDWHILE: JUMP WHILE; END_WHILE: // First instruction past loop 5/22/2021 Building a simple loop using Blackfin assembly code Copyright M. Smith, University of Calgary, Canada 16 / 27

Code the do-while loop sum_R 0 = 0; ? ? DO_WHILE: DO_WHILE int counter = 0; do { sum += counter; counter++; } while (counter <= 15); } 5/22/2021 // Loop body sum_R 0 = sum_R 0 + counter_R 1; ? ? END_DO_WHILE: // First instruction past loop Building a simple loop using Blackfin assembly code Copyright M. Smith, University of Calgary, Canada 17 / 27

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) 5/22/2021 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 18 / 27

Code the do-while loop with initial test DO_WHILE WITH INITIAL TEST unsigned short int counter = 0; if (counter > time. To. Use) { do { counter++; } while (counter <= time. To. Use) } 5/22/2021 Building a simple loop using Blackfin assembly code Copyright M. Smith, University of Calgary, Canada 19 / 27

Questions to answer at a later time Speed -- timing issues n n 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 5/22/2021 Building a simple loop using Blackfin assembly code Copyright M. Smith, University of Calgary, Canada 20 / 27

Major problem n A major problem with any form of loop is the “one-off” problem ¡ ¡ n 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? 5/22/2021 Building a simple loop using Blackfin assembly code Copyright M. Smith, University of Calgary, Canada 21 / 27

Blackfin “real” for-loops Needed in Assignment 1 #define MAX_LOOP 26 int array[MAX_LOOP]; int sum = 0; for (counter = 0; count <= MAX_LOOP; count++) { sum += array[counter]; } n 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 5/22/2021 Building a simple loop using Blackfin assembly code Copyright M. Smith, University of Calgary, Canada 22 / 27

Blackfin “real” for-loops Needed in Assignment 1 #define MAX_LOOP 26 int array[MAX_LOOP]; int sum = 0; for (counter = 0; count <= MAX_LOOP; count++) { sum += array[counter]; } n 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 hardware registers Loop_Top LT 1 and LT 0 (address) Loop_Bottom LB 1 and LB 0 (address) Loop_Count LC 1 and LC 0 (number) “Real” assembly language loops not found on all processors 5/22/2021 Building a simple loop using Blackfin assembly code Copyright M. Smith, University of Calgary, Canada 23 / 27

Code the “big loop” format #define MAX_LOOP 26 int array[MAX_LOOP]; int sum = 0; for (counter = 0; count <= MAX_LOOP; count++) { sum += array[counter]; } 5/22/2021 Form 1 – big loops LOOP loop_name loop_counter LOOP_BEGIN loop_name LOOP_END loop_name MORE DETAILS LATER Building a simple loop using Blackfin assembly code Copyright M. Smith, University of Calgary, Canada 24 / 27

Code the “small loop” format #define MAX_LOOP 26 int array[MAX_LOOP]; int sum = 0; for (counter = 0; count <= MAX_LOOP; count++) { sum += array[counter]; } 5/22/2021 Form 2 – small loops LSETUP (Begin_Loop, End_Loop) loop_counter MORE DETAILS LATER Building a simple loop using Blackfin assembly code Copyright M. Smith, University of Calgary, Canada 25 / 27

Tackled Today n n n Differences in behaviour between for-loops, while-loops, do-while loops with initial test Conditional JUMP and Conditional MOVE instructions Setting the CC condition code register ¡ ¡ 5/22/2021 What you would like to do, and can What you would like to do, but can’t Building a simple loop using Blackfin assembly code Copyright M. Smith, University of Calgary, Canada 26 / 27

n Information taken from Analog Devices On-line Manuals with permission http: //www. analog. com/processors/resources/technical. Library/manuals/ n 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. 5/22/2021 Building a simple loop using Blackfin assembly code Copyright M. Smith, University of Calgary, Canada 27 / 27