PC-technology

MASM and Inline Code in C

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MASM and Inline Code in C

• Some things to think about– Which size has the register you use

• AX, AH, AL• MOV AX, 500 Max 16 bits• MOV AH, 5 Max 8 bits• MOV AL,2 Max 8 bits• Now AX holds 0502H (1282 decimal)

– OUT always uses the accumulator, AX• in older versions of processors, only lower

byte, AL

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MASM and Inline Code in C

• Some things to think about, cont.– MUL always works on AX

• MUL BL BL*AL the result in AX• MUL BX BX*AX the result in DX:AX

– DX*65536+AX because DX holds the 16 higher bits and AX the lower 16 bits

– When working with .exe-files, the code and data have to be placed in separate segments

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One solution to the Organ

• Make an array of the numbers to divide the clock frequency with:– TONE DW 0, 0E24h, 11D1h, 0, 0,

10D0h, 0EFBh,0 – The first one is used when character A

is pressed, the next, the 0E24H when B is pressed, and 11D1H gives the tone C when C is pressed.

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One solution to the Organ• How do you get the right address?

– Ex. You press the C and want the tone C to sound. C has the ASCII code 67 and A has the code 65• If you take ‘A’-65 you get 0• If you take ‘C’-65 you get 2• When you go into the array the first number in the array is

at address TONE + 0, the next at address TONE+2, the next at TONE+4 => if C is pressed you want to goto address 2*2 which is (‘C’-65)*2

MOV BH,0 ;empty high byte of BXSUB BX,'A' ;ASCII-code - 65ADD BX,BX ; (ASCII code-65) * 2

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One solution to the Organ

• How do you get the right address?– To point at address TONE:

• MOV SI,OFFSET TONE

– To point at the first byte of number corresponding to ‘C’

– MOV AL,DS:[SI+BX]

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Inline Code in C

• Links to get help with Inline code:– http://www.microsoft.com/

• in Swedish: http://www.microsoft.com/sverige/

– use the search function on Inline Assembly

• Links to other assembly language topics• http://www.jorgon.freeserve.co.uk/• http://home6.swipnet.se/~w-61969/asm/ASM-

NY.htm (in Swedish)• http://www.jegerlehner.ch/intel/• http://brianvictor.8m.com/

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Inline Code in C

• Usage– need a piece of program that runs

faster than the C-code– and want to use C code to type things

onto the screen– want to use the data segment in the

same way as in C

• Examples at MSDN

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# include <iostream.h>void main(void){

unsigned short number1, number2, regAX, regDX;long int product;number1=500;number2=500;_asm{

mov ax,number1mov bx,number2mul bxmov regAX,axmov regDX,dx

}cout<<regAX<<endl;cout<<regDX<<endl;product = regAX+65536*(long)regDX;cout<<product;

}

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/* POWER2.C */#include <stdio.h>int power2( int num, int power );void main( void ){ printf( "3 times 2 to the power of 5 is %d\n", \power2( 3, 5) );}int power2( int num, int power ){ __asm { mov eax, num ; Get first argument mov ecx, power ; Get second argument shl eax, cl ; EAX = EAX * ( 2 to the power of CL ) } /* Return with result in EAX */}