assembly language review

Assembly Language Review

Being able to repeat on the Blackfin the things we were able to do on the MIPS

04/20/23 Review of 50% OF ENCM369 in 50 minutes 1

Assembly code things to review50% of ENCM369 in 50 minutesYOU ALREADY KNOW HOW TO DO THESE THINGS ON THE MIPS Being able to ADD and SUBTRACT the contents of two data

registers Being able to bitwise AND and bitwise OR the contents of two data

registers Being able to place a (small) required value into a data register Being able to place a (large) required value into a data register Being able to write a simple “void” function (returns nothing) Being able to write a simple “int” function (returns and int) Being able to ADD and SUBTRACT the contents of two memory

locations IF YOU CAN DO THE SAME THING ON THE BLACKFIN – THEN

THAT’S 50% OF THE LABS AND 50% OF EXAMS ACED

04/20/23 2 / 28

Being able to ADD and SUBTRACT the contents of two data registers Blackfin DATA registers R0, R1, R2 and R3

R0 = R1 + R2; // Addition R3 = R1 – R2; // Subtraction

It makes sense to ADD and SUBTRACT “values” stored in data registers

04/20/23 Review of 50% OF ENCM369 in 50 minutes 3 / 28

Being able to bitwise AND and OR the contents of two data registers Blackfin DATA registers R0, R1, R2 and R3

R0 = R1 & R2; // Bitwise AND R3 = R1 | R2; // Bitwise OR

It makes sense to perform OR and AND operations on “bit-patterns” stored in data registers.

NEVER perform ADD and SUBTRACT operations on “bit-patterns” stored in data registers. (Although SOMETIMES get the correct answer – code defect)


Is it a bit pattern or a value?Hints from “C++”If the code developer is consistent when writing the code then Bit patterns are normally stored as “unsigned integers” e.g.

unsigned int bitPattern = 0xFFFFFFFF

Values are normally stored as “signed integers” e.g.

signed int fooValue = -1; or int fooValue = -1; where the word “signed” is “understood”.Understood means “its there but not actually written down” (which means that it sometimes causes defects in your code)

Note that “bitPattern = 0xFFFFFFFF” and “fooValue = -1” are stored as the SAME bit pattern 0xFFFFFFFFF in the registers and memory of MIPS and Blackfin processor


Being able to place a required value into a data register –1 Like the MIPS, the Blackfin uses 32 bit instructions –

all registers are the same size to ensure maximum speed of the processor (highly pipelined instructions).

The 32 bit Blackfin instruction for placing a value into a data register has two parts to have16 bits available for describing the instruction and 16 bits for describing the “signed” 16 bit value to

be put into a “signed” 32 bit data register. This means that you have to use “2” 32-bit

instructions to put large values into a data register (SAME AS MIPS).


Placing a value into a data register Similar to MIPS, different syntax R1 = 0; legal -- 0 = 0x0000 (signed 16 bits); (becomes the signed 32 bit 0x00000000

after auto sign extension of the 16-bit value 0x0000)

R0 = 33; legal -- 33 = 0x0021 (signed 16 bits) (becomes the signed 32 bit 0x00000021

after auto sign extension of the 16-bit value 0x0021)

R2 = -1; legal -- -1 = 0xFFFF (signed 16 bits) (becomes the signed 32 bit 0xFFFFFFFF

after auto sign extension of the 16-bit value 0xFFFF)

R3 = -33; legal -- -33 = 0xFFDE (signed16 bits) (becomes the signed 32 bit 0xFFFFFFDE

after auto sign extension of the 16-bit value 0xFFDE)


Placing a “large” value into a data register This approach does not work for any “large” value

R1 = 40000; DOES NOT WORK WITH MIPS EITHER

illegal -- as 40000 can’t be expressed as a signed 16-bit value – it is the positive 32 bit value 0x00009C40

If the assembler tried to take the bottom 16 bits of the decimal 40000 and sign extend it then this would happen “16-bit” hex value 9C40 (1001 1100 0100 0000) becomes “32-bit” hex value after sign extension 0xFFFF9C40 which is a “negative value”

Therefore it is “illegal” to try to put a 32-bit value directly into a register; just as it would be illegal to try in MIPS.


Placing a “large” value into a data register If the assembler tried to take the bottom 16 bits of the decimal 40000 and sign

extend it then this would happen “16-bit” hex value 9C40 (1001 1100 0100 0000) becomes “32-bit” hex value after sign extension 0xFFFF9C40 which is a “negative value”

“illegal” just as it would be in MIPS

// Want to do R1 = 40000

// Instead must do operation in two steps as with MIPS

#include <blackfin.h>

R1.L = lo(40000); // Tell assembler to put “bottom”

// 16-bits into “low” part of R1 register

R1.H = hi(40000); // Tell assembler to put “top”

// 16-bits into “high” part of R1 register

04/20/23 9 / 28

Placing a “large” value into a data register

A common error in the laboratory and exams is getting this two step thing “wrong” . Forgetting the second step is easy to do – just as easy to forget on Blackfin as on MIPS

// Want to do R1 = 41235

R1.L = lo(41235); // “bottom” 16-bits into “low” part of R1 register

R1.H = hi(41325); // “top” 16-bits into “high” part of R1 register FORGOTTEN SECOND STEP

RECOMMENDED SYNTAX TO AVOID “CODE DEFECTS”#define LARGEVALUE 41235 // C++ - like syntax

R1.L = lo(LARGEVALUE); R1.H = hi(LARGEVALUE);Yes – you CAN put multiple Blackfin assembly language instructions on one line


A “void” function returns NO VALUEextern “C” void SimpleVoidASM(void)


.section program;

.global _SimpleVoidASM;

_SimpleVoidASM:

_SimpleVoidASM.END: RTS;


Things in red were cut-and-pasted using the editor

to save Lab. time

A simple “int” function return a valueextern “C” int SimpleIntASM(void)


.section program;

.global _SimpleIntASM;

_SimpleIntASM:

R0 = 7; // Return “7”

_SimpleIntASM.END: RTS;



Being able to ADD and SUBTRACT the contents of two memory locations

Let’s set up a practical situation A “background” thread is putting values into an

array. Processor could be MIPS or Blackfin For “background” thread read

“interrupt service routine” or ISR. ISR work “in parallel” with the “foreground” thread

that is doing the major work on the microprocessor

Write a subroutine (returns int) that adds together the first two values of this shared array


Start with a copy of the “int” function extern “C” int SimpleIntASM(void)


.section program;

.global _SimpleIntASM;

_SimpleIntASM:

R0 = 7; // Return “7”

_SimpleIntASM.END: RTS;



Modify to be extern “C” int AddArrayValuesASM(void)


.section program;

.global _AddArrayValuesASM;

_AddArrayValuesASM:

R0 = 7; // Return “7”

_AddArrayValuesASM.END: RTS;



Add a “data” array


.section L1_data;

.byte4 _fooArray[2]; // Syntax for building an array

// of 32-bit values

.section program;


_AddArrayValuesASM :

R0 = 7; // Return “7”

_AddArrayValuesASM .END: RTS;



Plan to return “sum”, initialize sum to 0


.section L1_data;

.byte4 _fooArray[2];

.section program;


_AddArrayValuesASM:

#define sum_R0 R0 // register int sum;

sum_R0 = 0; // sum = 0;




Place the memory address of the start of the array into a pointer register…. Other code

.section L1_data;


.section program;




sum_R0 = 0; // sum = 0;

#define pointer_to_array_P1 P1 // register int * pointer_to_array

P1.L = lo(_fooArray); P1.H = hi(_fooArray);

// pointer_to_array = &fooArray[0];




P1 is a POINTER register(address register)

Read the contents of the first array location into register R1 and add to sum_R0;

…. Other code

.section L1_data;


.section program;




sum_R0 = 0; // sum = 0;

#define pointer_to_array_P1 P1 // register int * pointer_to_arrayP1L = lo(_fooArray); P1.H = hi(_fooArray); // pointer_to_array = &fooArray[0];

R1 = [pointer_to_array_P1]; // int temp = fooArray[0]; sum_R0 = sum_R0 + R1; // sum = sum + temp

_AddArrayValuesASM.END: RTS;04/20/23 Review of 50% OF ENCM369 in 50 minutes 19 / 28


Read the contents of the second array location into register R1 and add to sum_R0;

…. Other code

.section L1_data;


.section program;


_AddArrayValuesASM:


sum_R0 = 0; // sum = 0;

#define pointer_to_array_P1 P1 // register int * pointer_to_arrayP1.L = lo(_fooArray); P1.H = hi(_fooArray); // pointer_to_array = &fooArray[0];R1 = [pointer_to_array_P1]; // int temp = fooArray[0]; sum_R0 = sum_R0 + R1; // sum = sum + temp

R1 = [pointer_to_array_P1 + 4]; // temp = fooArray[1]; sum_R0 = sum_R0 + R1; // sum = sum + temp

_AddArrayValuesASM .END: RTS;04/20/23 20 / 28


Add code to .ASM (assembly) file


Assignment 1, Q1Demo answer


Assembly code things to review50% of ENCM369 in 50 minutes

YOU ALREADY KNOW HOW TO DO THESE THINGS ON THE MIPS Being able to ADD and SUBTRACT the contents of two data

registers Being able to bitwise AND and bitwise OR the contents of two data

registers Being able to place a (small) required value into a data register Being able to place a (large) required value into a data register Being able to write a simple “void” function (returns nothing) Being able to write a simple “int” function (returns and int) Being able to ADD and SUBTRACT the contents of two memory

locations IF YOU CAN DO THE SAME THING ON THE BLACKFIN – THEN

THAT’S 50% OF THE LABS AND 50% OF EXAMS ACED

04/20/23 23 / 28

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