High-Level Language Interface

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High-Level Language Interface

Computer Organization and Assembly Languages Yung-Yu Chuang

2005/12/15

with slides by Kip Irvine

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Overview

• Why Link ASM and HLL Programs?

• Inline Assembly Code

• Linking to C++ Programs

• Optimizing Your Code

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Why link ASM and HLL programs?

• Use high-level language for overall project development

– Relieves programmer from low-level details

• Use assembly language code

– Speed up critical sections of code

– Access nonstandard hardware devices – Write platform-specific code

– Extend the HLL's capabilities

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General conventions

• Considerations when calling assembly language procedures from high-level languages:

– Both must use the same naming convention (rules regarding the naming of variables and procedures) – Both must use the same memory model, with

compatible segment names

– Both must use the same calling convention

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Calling convention

• Identifies specific registers that must be preserved by procedures

• Determines how arguments are passed to

procedures: in registers, on the stack, in shared memory, etc.

• Determines the order in which arguments are passed by calling programs to procedures

• Determines whether arguments are passed by value or by reference

• Determines how the stack pointer is restored after a procedure call

• Determines how functions return values

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External identifiers

• An external identifier is a name that has been placed in a module’s object file in such a way that the linker can make the name available to other program modules.

• The linker resolves references to external identifiers, but can only do so if the same naming convention is used in all program modules.

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Inline assembly code

• Assembly language source code that is inserted directly into a HLL program.

• Compilers such as Microsoft Visual C++ and

Borland C++ have compiler-specific directives that identify inline ASM code.

• Efficient inline code executes quickly because CALL and RET instructions are not required.

• Simple to code because there are no external names, memory models, or naming conventions involved.

• Decidedly not portable because it is written for a single platform.

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_asm directive in Microsoft Visual C++

• Can be placed at the beginning of a single statement

• Or, It can mark the beginning of a block of assembly language statements

• Syntax: ^__asm ^statement

__asm {

statement-1 statement-2 ...

statement-n }

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Commenting styles

mov esi,buf ; initialize index register mov esi,buf // initialize index register mov esi,buf /* initialize index register*/

All of the following comment styles are acceptable, but the latter two are preferred:

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You can do the following . . .

• Use any instruction from the Intel instruction set

• Use register names as operands

• Reference function parameters by name

• Reference code labels and variables that were declared outside the asm block

• Use numeric literals that incorporate either assembler-style or C-style radix notation

• Use the PTR operator in statements such as inc BYTE PTR [esi]

• Use the EVEN and ALIGN directives

• Use LENGTH, TYPE, and SIZE directives

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You cannot do the following . . .

• Use data definition directives such as DB, DW, or BYTE

• Use assembler operators other than PTR

• Use STRUCT, RECORD, WIDTH, and MASK

• Use macro directives such as MACRO, REPT, IRC, IRP

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Register usage

• In general, you can modify EAX, EBX, ECX, and EDX in your inline code because the compiler does not expect these values to be preserved between statements

• Conversely, always save and restore ESI, EDI, and EBP.

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File encryption example

• Reads a file, encrypts it, and writes the output to another file.

• The TranslateBuffer function uses an __asm

block to define statements that loop through a character array and XOR each character with a predefined value.

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TranslateBuffer

void TranslateBuffer(char * buf,

unsigned count,

unsigned char eChar ) {

__asm {

mov esi,buf ; set index register mov ecx,count /* set loop counter */

mov al,eChar L1:

xor [esi],al inc esi

Loop L1 } // asm

}

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File encryption

while (!infile.eof() ) {

infile.read(buffer, BUFSIZE );

count = infile.gcount();

TranslateBuffer(buffer, count, encryptCode);

outfile.write(buffer, count);

}

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File encryption

while (!infile.eof() ) {

infile.read(buffer, BUFSIZE );

count = infile.gcount();

__asm {

lea esi,buffer mov ecx,count

mov al, encryptChar L1:

xor [esi],al inc esi

Loop L1 } // asm

outfile.write(buffer, count);

}

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Linking assembly language to C++

• Basic Structure - Two Modules

– The first module, written in assembly language, contains the external procedure

– The second module contains the C/C++ code that starts and ends the program

• The C++ module adds the extern qualifier to the external assembly language function

prototype.

• The "C" specifier must be included to prevent name decoration by the C++ compiler:

extern "C" functionName( parameterList );

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Name decoration

Also known as name mangling. HLL compilers do this to uniquely identify overloaded functions. A function such as:

int ArraySum( int * p, int count )

would be exported as a decorated name that encodes the return type, function name, and parameter types. For

example:

int_ArraySum_pInt_int

The problem with name decoration is that the C++ compiler assumes that your assembly language function's name is decorated. The C++ compiler tells the linker to look for a decorated name.

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• The 90/10 rule: 90% of a program's CPU time is spent executing 10% of the program's code

• We will concentrate on optimizing ASM code for speed of execution

• Loops are the most effective place to optimize code

• Two simple ways to optimize a loop:

– Move invariant code out of the loop

– Substitute registers for variables to reduce the number of memory accesses

– Take advantage of high-level instructions such as XLAT, SCASB, and MOVSD.

Optimizing Your Code

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Loop optimization example

.data

days DWORD ?

minutesInDay DWORD ? totalMinutes DWORD ?

str1 BYTE "Daily total minutes: ",0

• We will write a short program that calculates and displays the number of elapsed minutes, over a period of n days.

• The following variables are used:

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Sample program output

Daily total minutes: +1440 Daily total minutes: +2880 Daily total minutes: +4320 Daily total minutes: +5760 Daily total minutes: +7200 Daily total minutes: +8640 Daily total minutes: +10080 Daily total minutes: +11520 .

.

Daily total minutes: +67680 Daily total minutes: +69120 Daily total minutes: +70560 Daily total minutes: +72000

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No optimization.

mov days,0

mov totalMinutes,0

L1: ; loop contains 15 instructions mov eax,24 ; minutesInDay = 24 * 60 mov ebx,60

mul ebx

mov minutesInDay,eax

mov edx,totalMinutes ; totalMinutes += minutesInDay add edx,minutesInDay

mov totalMinutes,edx

mov edx,OFFSET str1 ; "Daily total minutes: "

call WriteString

mov eax,totalMinutes ; display totalMinutes call WriteInt

call Crlf

inc days ; days++

cmp days,50 ; if days < 50,

jb L1 ; repeat the loop

Version 1

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Move calculation of minutesInDay outside the loop, and assign EDX before the loop. The loop now contains 10 instructions.

mov days,0

mov totalMinutes,0

mov eax,24 ; minutesInDay = 24 * 60 mov ebx,60

mul ebx

mov minutesInDay,eax

L1:mov edx,totalMinutes ; totalMinutes += minutesInDay add edx,minutesInDay

mov totalMinutes,edx

call WriteString ; display str1 (offset in EDX) mov eax,totalMinutes ; display totalMinutes

call WriteInt call Crlf

cmp days,50 ; if days < 50,

Version 2

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Move totalMinutes to EAX, use EAX throughout loop. Use constant expresion for minutesInDay calculation. The loop now contains 7 instructions.

C_minutesInDay = 24 * 60 ; constant expression mov days,0

mov totalMinutes,0 mov eax,totalMinutes

L1:add eax,C_minutesInDay ; totalMinutes+=minutesInDay call WriteString ; display str1 (offset in EDX) call WriteInt ; display totalMinutes (EAX) call Crlf

cmp days,50 ; if days < 50,

mov totalMinutes,eax ; update variable

Version 3

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Substitute ECX for the days variable. Remove initial assignments to days and totalMinutes.

C_minutesInDay = 24 * 60 ; constant expression mov eax,0 ; EAX = totalMinutes

mov ecx,0 ; ECX = days

L1:; loop contains 7 instructions

add eax,C_minutesInDay ; totalMinutes+=minutesInDay call WriteString ; display str1 (offset in EDX) call WriteInt ; display totalMinutes (EAX) call Crlf

inc ecx ; days (ECX)++

cmp ecx,50 ; if days < 50,

mov totalMinutes,eax ; update variable mov days,ecx ; update variable

Version 4

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Using assembly to optimize C++

• Find out how to make your C++ compiler

produce an assembly language source listing

– /FAs command-line option in Visual C++, for example

• Optimize loops for speed

• Use hardware-level I/O for optimum speed

• Use BIOS-level I/O for medium speed

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FindArray example

#include "findarr.h"

bool FindArray( long searchVal, long array[], long count )

{

for(int i = 0; i < count; i++) if( searchVal == array[i] )

return true;

return false;

}

Let's write a C++ function that searches for the first matching integer in an array. The function returns true if the integer is found, and false if it is not:

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Code produced by C++ compiler

_searchVal$ = 8 _array$ = 12 _count$ = 16 _i$ = -4

_FindArray PROC NEAR

; 29 : { push ebp

mov ebp, esp push ecx

; 30 : for(int i = 0; i < count; i++) mov DWORD PTR _i$[ebp], 0

jmp SHORT $L174

$L175:

mov eax, DWORD PTR _i$[ebp]

add eax, 1

mov DWORD PTR _i$[ebp], eax

optimization switch turned off (1 of 3)

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Code produced by C++ compiler

$L174:

mov ecx, DWORD PTR _i$[ebp]

cmp ecx, DWORD PTR _count$[ebp]

jge SHORT $L176

; 31 : if( searchVal == array[i] ) mov edx, DWORD PTR _i$[ebp]

mov eax, DWORD PTR _array$[ebp]

mov ecx, DWORD PTR _searchVal$[ebp]

cmp ecx, DWORD PTR [eax+edx*4]

jne SHORT $L177

; 32 : return true;

mov al, 1

jmp SHORT $L172

$L177:

; 33 :

; 34 : return false;

jmp SHORT $L175

(2 of 3)

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Code produced by C++ compiler

$L176:

xor al, al ; AL = 0

$L172:

; 35 : }

mov esp, ebp ; restore stack pointer pop ebp

ret 0

_FindArray ENDP

(3 of 3)

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Hand-coded assembly language

true = 1 false = 0

; Stack parameters:

srchVal equ [ebp+08]

arrayPtr equ [ebp+12]

count equ [ebp+16]

.code

_FindArray PROC near push ebp

mov ebp,esp push edi

mov eax, srchVal ; search value

mov ecx, count ; number of items mov edi, arrayPtr ; pointer to array

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Hand-coded assembly language

repne scasd ; do the search jz returnTrue ; ZF = 1 if found returnFalse:

mov al, false jmp short exit

returnTrue:

mov al, true exit:

pop edi pop ebp ret

_FindArray ENDP