1. Hints for Post-Lab Quiz 1
Works for other quizzes and exams too

2. Three types of questions
Basic Knowledge Questions
Translation Questions
You are given the C++ and must convert into Blackfin assembly code
Design questions
Work out what is needed
Generate the design – in C++ or in pseudo code
Most often – convert design code into Blackfin assembly code

3. Knowledge type question example
CORRECT OR WRONG ANSWER NO PARTIAL MARKS
Sometimes more than one correct answer
Circle and label with an A -- the icon (menu item) that causes VisualDSP to compile the C++ code, but not build or rebuild the full project.
B) Circle and label with a B -- a Blackfin instruction where a non-volatile register is recovered from the stack.

4. The Rosetta Stone “Question” You understand columns 1 and 2
Demonstrates ability to transfer knowledge Which register is used as the 68K return register? In this code, which register is used as the 68K frame pointer?
C++ code
NEW C++ keyword
extern “C” long int Return1( );
long int Return1( ){ return 1; }
Blackfin code
.section program; global _Return1;
_Return1: LINK 16;
R0 = 1;
UNLINK; RTS;
68K code
.section code; global _Return1;
_Return1: LINK #-16, A4;
MOVE.L #1, D0;
UNLINK A4; RTS;

5. C++ to assembly Example
ACCURACY IMPORTANT
#define count_R1 R1 count_R1 = 0;
TRY TO MATCH ASSEMBLY CODE TO C++ ON A BOX BY BOX BASIS
THEN EASIER TO GIVE PARTIAL MARKS

6. Design question example Acts like one part of the laboratory

7. Design Question Next to impossible to mark if not well documented
Therefore many marks are given for the C++ or pseudo-code comments
More chance of partial marks if the register names are self documenting

8. Register documentation example
ID_R1.L = lo(CHIPID); // Marker know that ID_R1.H = hi(CHIPID); // R1 used to store ID
CC = ID_R1 == version_INPAR_R0; // Marker knows that // R0 used for version // Marker also know that you know first parameter is passed in R0
// and not on the stack – later if you make a mistake version_R1 then still a good chance for partial (or full) mark

9. Avoid errors that would take a lot of time to fix in the laboratory
Always check for possible return address and stack errors
LINK at the start of a function
UNLINK at the end of a function
Always check for NAME_MANGLING
Variable _fooarray;
Function _FeeFunction__Fv (void)
_FeeFunction__Fl (long int) _FeeFunction__NM (not sure) _FeeFunction__NM2 (different not sure)
WITH NAME MANGLING – under exam conditions, more interested in that you understand the concept than whether you are getting it exactly correct
We are using extern “C” to this point in the class. We will get to “function overloading” and “name_mangling” at a latter date

10. Avoid pointer errors that would take a lot of time to fix in the laboratory
If the memory location is shown as extern in C++ or .extern in Assembly
extern long int funVariable;
.extern _funVariable;
.section program // will accept .section code
P0.L = lo(_funVariable); P0.H = hi(_funVariable);
_funInRegisterR0 = [P0];

11. Avoid pointer errors that would take a lot of time to fix in the laboratory
If the memory location is shown without the word EXTERN
long int funVariable = 0;
.section L1_data1; // will accept data, data1
.global _funVariable;
.var _funVariable = 0; // Follow the C++ code funVariable is IN MEMORY and not yet in a register
You must move the value from memory into a register
.section program
P0.L = lo(_funVariable); IS P0.L = _funVariable
P0.H = hi( _funVariable); OKAY? funInRegisterR0 = [P0];

12. Avoid pointer errors that would take a lot of time to fix in the laboratory
If the memory location is known to be part of the special MEMORY LOCATIONS (MMR) used to control special operations of the Blackfin “peripherals”
#include <blackfin.h>
.section program; // Will note accept missing sections
P0.L = lo(TCOUNT); // will accept HI( ) and LO ( ) P0.H = hi(TCOUNT); countInRegisterR0 = [P0];

13. Know what the hi( ) and lo( ) macros do
.section program P0.L = lo(TCOUNT); P0.H = hi(TCOUNT); MEANS P0.L = TCOUNT & 0xFFFF; P0.H = (TCOUNT & 0xFFFF0000) >>16

14. HINT – #define value CONSTANTS don’t use CONSTANTS (magic numbers)
#define MAXVALUE Either hex or decimal is okay
.section program
R0.L = lo(MAXVALUE); R0.H = hi(MAXVALUE);
HINT: If the person is following “standard” coding conventions then CAPITIALS MEAN CONSTANT – use hi(), lo( )

15. HINT – Will work for small constants too
#define MAXVALUE Either hex or decimal is okay .section program R0.L = lo(MAXVALUE); R0.H = hi(MAXVALUE); BUT IN THIS CASE – since the constant is small (short int size) R0 = MAXVALUE; Or R0 = 6; HINT: If it looks like IT MIGHT BE a big constant, then let the assembler worry about it -- use hi( ) and lo( )

16. Condition codes 99999 times out of 100000 the following is wrong
CC = R0 < number; e.g. CC = R0 < 25;
So play the odds
R1 = number;
CC = R0 < R1;
Will accept CC = (R0 < R1); under exam conditions – extra brackets
WILL NOT ACCEPT CC = R1 > R0;
CC conditions are always checked VERY closely as they cause so much problem in the laboratory and in “real life”

17. LOAD AND STORE OPERATIONS
Rule to remember – if the operation would not work on the MIPS, then it will not work on the Blackfin or any other RISC processor
register  memory R0 = [P1]; memory  register [P1] = R0;
NEVER add to memory, [P1] = [P0] +1; add to register R0 = R0 + [P0];

18. Register operations Add a small number
Make sure that you get the common instructions correct – there are not many
R0 += pretty_small_number R0 += or R0 += -10; NOT R0 = R0 + 6;
Pretty_small_numbers are just that – pretty small numbers <= num <= +63

19. Register operations Add a larger number
Make sure that you get the common instructions correct – there are not many instructions that you need to be concerned with
R1 = larger_number;
R0 = R0 + R1;
R1 = 0x2000;
R0 = R0 + R1; NOT R0 += R1; R1 = 20000; R0 = R0 + R1;
R1.L = lo(40000); R1.H = hi(40000); R0 = R0 + R1;
HINT: Hexadecimal numbers are easy to work out if they are small (need 16-bits) or very large (need 32- bits). Decimal numbers are not – PLAY THE ODDS – if it looks large in decimal – then use lo( ), hi( ) approach

20. Other instructions we have used
Make sure that you get the common instructions correct – there are not many common instructions to worry about
JUMP LABEL_END; // OFTEN JUMP (P0); // typically end of function

21. Other instructions we have used
Make sure that you get the common instructions correct – there are not many
CALL _FeePassVoidFunction__Fv // void FeePassVoidFunction(void);
NOTE: CALL _FeePassVoidFunction__Fv // long int FeePassVoidFunction(void); // Returns a value in R0;
extern “C” _FeePassVoidFunction – used in this class
CALL _FeePassVoidFunction

22. Other instructions we have used
Make sure that you get the common instructions correct – there are not many
// void FeePassLongIntFunction(long int);
CALL _FeePassLongIntFunction__Fl (little L) CALL _FeePassLongIntFunction__NM -- okay in exam
CALL _FeePassIntFunction__Fi (little I) // void FeePassIntFunction(long int);
CALL _FeePassIntFunction__NM okay in exam

23. Other instructions we have used
Make sure that you get the common instructions correct – there are not many
R0 = 7; CALL _FeeFunction__Fl; // FeeFunction( 7);
R1 = 6; R0 = 7; CALL _FumFunction__NM; // FumFunction(7, 6 );

24. Other instructions we have used
Make sure that you get the common instructions correct – there are not many
R0 = 7; CALL _FeeFunction__Fl; // FeeFunction( 7);
R1 = 6; R0 = 7; CALL _FumFunction__NM; // FumFunction(7, 6 );

25. When to use a register and when to use memory
.extern _value; // extern long int value section L1_data; global _fum_value; // long int fum_value;
.var _fum_value;
.section program; global _FooFunction__Fl; // void FooFunction(long int passed_Fickle) {
_FooFunction__Fl: LINK 16; passed_Fickle_R0 += 6; // passed_Fickle = passed_Fickle +6; P0.H = hi(_value); P0.L = lo(_value); // value = value + 6; R1 = [P0]; R1 += 6;
[P0] = R1;
P1.H = hi(_fum_value); P1.L = lo(_fum_value); // fum_value = fum_value + 6; R2 = [P1]; R2 += 6;
[P1] = R2;
……… // Rest of the function

26. When to use a register and when to use memory
.section program; .global _FooFunction__Fl; // void FooFunction(long int passed_Fickle) { _FooFunction__Fl: LINK 16; passed_Fickle_R0 += 6; // passed_Fickle = passed_Fickle +6; #define value_R1 R1 // long int value value_R1 += 6; // value = value + 6; #define fum_valueR2 R2 // long int fum_value; fum_value_R2 += 6; // fum_value = fum_value + 6; ……… // Rest of the function

27. Other requested question and answers