Assembly-Language Expressions

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02/04/2013

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The debugger can correctly evaluate assembly-language expressions, with some restrictions. The syntax used for some assembly-language expressions differs from the syntax used in assembly-language development systems, such as the Microsoft Macro Assembler (MASM).

Memory Operators

Memory operators are unary operators that return the result of a direct memory operation. These operators are used mainly to debug assembly-language code.

{BY | WO | DW} address

The BY operator returns a short integer that contains the first byte at address. This operator simulates BYTE PTR.

The WO operator returns a short integer that contains the value of the word, two bytes, at address. This operator simulates the Microsoft Macro Assembler WORD PTR operation. The DW operator returns a long integer that contains the value of the first four bytes at address. This operator simulates DWORD PTR.

The x format specifier used in some of these examples causes the result to be displayed in hexadecimal.

Examples

To display the first byte at the address of the variable sum:

BY sum
To display the first word at the address of the variable new_set:

WO new_set
To display the doubleword at the address of sum:

DW sum
To display the byte pointed to by the EBP register with a displacement of 6:

BY ebp+6,x
To display the word pointed to by the stack pointer, the last word pushed onto the stack:

WO esp,x
To display the doubleword pointed to by the ESI register:

DW esi,x

Register Indirect

The debugger does not recognize brackets ([ ]) to indicate a memory location pointed to by a register. Instead, use the BY, WO, and DW operators to reference the corresponding byte, word, or doubleword values.

MASM expression	Debugger expression	C++ expression
BYTE PTR [bx]	BY ebx	*(unsigned char) ebx
WORD PTR [bp]	WO ebp	(unsigned short ) ebp
DWORD PTR [bp]	DW ebp	(unsigned long ) ebp

Register Indirect with Displacement

To perform based, indexed, or based-indexed indirect mode operations with a displacement, use the BY, WO, or DW operators with the addition operator.

MASM expression	Debugger expression
BYTE PTR [edi+6]	BY edi+6
BYTE PTR Test[ebx]	BY &Test+ebx
WORD PTR [esi][ebp+6]	WO esi+ebp+6
DWORD PTR [ebx][esi]	DW ebx+esi

Address of a Variable

Use the C address-of operator (&) instead of the MASM OFFSET operator.

MASM expression	Debugger expression
OFFSET Var	&Var

PTR Operator

Use type casts or the BY, WO, and DW operators with the address-of operator (&) to replace the assembly-language PTR operator.

MASM expression	Debugger expression
BYTE PTR Var	BY &Var
(unsigned char)	&Var
WORD PTR Var	WO &Var
DWORD PTR Var	DW &Var
(unsigned long)	&Var

Assembly-Language Strings

Add the string format specifier ,s after the variable name.

MASM expression	Debugger expression
String	String,s

Because C strings end with a null (ASCII 0) character, the debugger displays all characters from the first byte of the variable up to the next null byte in memory when you request a string display. If you intend to debug an assembly-language program, and you want to view strings in the Watch window, you should delimit string variables with a null character. An easy way to view null-terminated or unterminated strings is by using the Memory window.

Array and Structure Elements

Prefix an array name with the address-of operator (&) and add the desired offset. The offset can be an expression, number, register name, or variable.

The following examples show how to do this for byte, word, and doubleword arrays.

MASM expression	Debugger expression
String[12]	BY &String+12*(&String+12)
aWords[bx+di]	WO &aWords+bx+di(unsigned)(&aWords+bx+di)
aDWords[bx+4]	DW &aDWords+bx+4(unsigned long)(&aDWords+bx+4)

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