assembly how to preserve and restore a register

Question

I wrote this code but my professor kept telling me to preserve and restore my registers. I thought I was by declaring a value to the empty registers.

program middleFinder;
#include( "stdlib.hhf" ); // imports the input and output library
static
iDataX : int16 := 0;
iDataY : int16 := 0;
iDataZ : int16 := 0;


procedure middleFindernow( x: int16; y : int16; z : int16 ); @nodisplay; @noframe; 
// this caller must free up DH
static
iReturnAddress : dword;
iMid : int16;
iTemporary : int16;
iRegisterValue : dword;


begin middleFindernow;


mov( EBX, iRegisterValue );
// acquire parameters on the stack
pop( iReturnAddress );
pop(iTemporary);
pop(iTemporary);
mov(iTemporary, BX);
mov( BX, z );
pop( iTemporary ); // this is Y
mov( iTemporary, BX );
mov( BX, y );
pop( iTemporary ); // this is X
mov( iTemporary, BX );
mov( BX, x );
// push back the return
push( iReturnAddress );
push( iRegisterValue );

// calculate x - y
mov(x, DX);
cmp(y, DX);
je XYequal;
cmp(y, DX);
jl YisMax;
cmp(y, DX);
jg XisMax;

XYequal:
mov(y, BX);
cmp(z, BX);
je equal;
jmp ExitSequence;

equal:
stdout.put("All three values are equal");
stdout.put("AL=1");
jmp ExitSequence;

XisMax:
// calculate x - z
mov(0,BX);
mov(z,BX);
cmp(x,BX);
jl PutXinDX;
jg PutZinDX;


YisMax:
// calculate y - z
mov(0, BX);
mov(y, BX);
cmp(z, BX); 
jl PutYinDX;
jg PutZinDX;


PutXinDX:
mov(x, DX);
jmp ExitSequence;

PutYinDX:
mov(y, DX);
jmp ExitSequence;

PutZinDX:
mov(z, DX);
jmp ExitSequence;

ExitSequence:
// exit sequence
// pop all preserved registers --- BX
pop( EBX );
ret();


end middleFindernow;
begin middleFinder;
stdout.put( "gimme a X value: " );
stdin.get( iDataX);
stdout.put( "gimme a Y value: " );
stdin.get( iDataY );
stdout.put( "gimme a Z value: " );
stdin.get( iDataZ ); 
stdout.put( "Calling Mid number " , nl );
mov( 0, BX );
mov( iDataX, BX );
push( BX );
mov( 0, BX );
mov( iDataY, BX );
push( BX );
mov( 0, BX );
mov( iDataZ, BX );
push( BX );
mov( 0, BX );
push( BX );
call middleFindernow;
stdout.put( "The value in the middle is " );
stdout.puti16( DX );
stdout.newln();

end middleFinder;

He's telling you to save the values of the registers during the function prologue and restore them in the epilogue. — David Hoelzer, Aug 13 '17 at 13:22

score 0 · Answer 1 · answered Aug 13 '17 at 06:15

0

You used some registers like BX but at the end you don't restore their initial values from before your program started, it may be what your professor meant

answered Aug 13 '17 at 06:15

Ilphrin

54
4

score 0 · Answer 2 · answered Aug 13 '17 at 08:51

I suppose professor want, that in the beginning of your functions you will push all registers, that function are changing to the stack and before return you will restore actual values of these registers. This is needed because code that will call your functions could use these values later and do not expect that values were changed.

assembly how to preserve and restore a register

2 Answers2