Hints for Post-Lab Quiz 1

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Hints for Post-Lab Quiz 1

• Works for other quizzes and exams too

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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

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Knowledge type question example
CORRECTOR WRONGANSWERNO PARTIAL
MARKS Sometimes more than one correct

• 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.

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The Rosetta StoneEngineering version
Demonstrates ability to transfer knowledgeWhich
register is used as the 68K return register?In
this code, which register is used as the 68K
frame pointer?
C code Used asextern 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
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C to assembly Example
ACCURACYIMPORTANT
define count_R1 R1count_R1 0
TRY TO MATCH ASSEMBLY CODE TO C ON A BOX BY BOX
BASIS THEN EASIER TO GIVEPARTIAL MARKS
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Design question exampleActs like one part of the
laboratory
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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 registers are
  self documenting

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Register documentation example
ID_R1.L lo(CHIPID) //
Marker know thatID_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_R1then still a good chanace for
partial (or full) mark
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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

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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 code
• P0.L _funVariableP0.H _funVariable

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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 data1 // will accept data, L1_data
• .global _funVariable
• .var _funVariable 0 // Follow the C
• .section program
• P0.L _funVariableP0.H _funVariable
• HINT If it is name mangled
  DONT use hi( ) or lo( )

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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 ltdefBF533.hgt // will accept ltdefs
• include ltmacros.hgt // will accept ltmacro.hgt
• .section program
• P0.L lo(TCOUNT) // will accept HI( ) and
  LO ( )P0.H hi(TCOUNT)
• HINT If it is name mangled
• DONT use hi( ) or lo( )

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HINT define CONSTANTSdont use CONSTANTS

• define MAXVALUE 44000 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( )

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HINT Will work for small constants too

• define MAXVALUE 22000 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( )

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Condition codes

• 99999 times out of 100000 the following is wrong
• CC R0 lt number
• So play the odds
• R1 number
• CC R0 lt R1
• Will accept CC (R0 lt R1)
• WILL NOT ACCEPT CC R1 gt R0
• CC conditions are always checked VERY closely as
  they cause so much problem in the laboratory and
  in real life

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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 P1memory ?
  register P1 R0
• NEVER add to memory, P1 P0 1 add
  to register R0 R0 P0

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Register operationsAdd a small number

• Make sure that you get the common instructions
  correct there are not many
• R0 pretty_small_number R0 6
  or R0 -10 NOT R0 R0 6
• Pretty_small_numbers are just that pretty small
  numbers -64 lt num lt 63

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Register operationsAdd a larger number

• Make sure that you get the common instructions
  correct there are not many
• 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

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Other instructions we have used

• Make sure that you get the common instructions
  correct there are not many
• JUMP LABEL_END // OFTEN JUMP
  (P0) // typically end of function

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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(vo
  id) // Returns a value in R0

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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__NM2 -- okay in exam

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Other instructions we have used

• Make sure that you get the common instructions
  correct there are not many
• R0 7 CALL _FeeFunction__Fl
  // FeeFunction( )
• R1 6 R0 7 CALL
  _FumFunction__NM //
  FumFunction(7, 6 )

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Other instructions we have used

• Make sure that you get the common instructions
  correct there are not many
• R0 7 CALL _FeeFunction__Fl
  // FeeFunction( )
• R1 6 R0 7 CALL
  _FumFunction__NM //
  FumFunction(7, 6 )

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When to use a register andwhen to use memory

• .extern _value // extern long int
  value .section L1_data_a .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
  _value // value
  value 6 P0.L _value R1 P0
  R1 6
• P0 R1
• P1.H _fum_value
  // fum_value fum_value 6 P1.L
  _fum_value R2 P1 R2 6
• P1 R2
• 
  // Rest of the function

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When to use a register andwhen 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

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• Other requested question and answers