Assuming x is a number smaller than 0xff what happens to x after the assignment LOBYTE(x)=x-1?

Question

I understand that LOBYTE is an IDA macro for retrieving the lower byte of a variable. My question is what is the difference between the result of x=x-1 and LOBYTE(x)=x-1 when x is smaller than or equal to 0xff? I should add that I'm implicitly assuming that x>0. Thank you!

Answer 1

Quote From Link     
#define LOBYTE(x)   (*((_BYTE*)&(x))) 

is that a hypothetical query x is treated as address
so x-1 will be a 32 bit type on a x86 machine so theoretically
you cannot assign a 32 bit type to an 8 bit type
LOBYTE(x) will be a byte and not an address so again theoretically you cannot assign a byte to a byte

LOBYTE(x) is an AND operation that extracts the unsigned byte from a specific address

x as address    contents         LOBYTE(x)     (byte *)&x = LOBYTE(x)-1
0x00400000      0xffffffff       0x000000ff    byte[0x004000000] = 0x000000ff -1 =0x000000fe

so if you look as a DWORD 0x400000 will now contain 0xfffffffe

demo using a python script

:\>cat LOBYTE.py
import ctypes

def LOBYTE(arg):
    return arg.value & 0x000000ff

x = ctypes.c_ulong(0xffffffff)
print( "x as address" , ctypes.byref(x))
print( "x holds"  , hex(x.value))
print("result of LOBYTE(x)", LOBYTE(x))
x.value = ( (x.value & 0xffffff00 ) | LOBYTE(x)- 1 )
print( "x holds"  , hex(x.value))


:\>python LOBYTE.py
x as address <cparam 'P' (017CA098)>
x holds 0xffffffff
result of LOBYTE(x) 255
x holds 0xfffffffe