x86, difference between BYTE and BYTE PTR

Question

What is the difference between these two lines? What PTR changes here?

;first
mov BYTE [ecx], 0  
;second
mov BYTE PTR [ecx], 0

Answer 1

In MASM, BYTE PTR [ecx] accesses memory at address ecx. BYTE [ecx] is a syntax error ("inline assembler syntax error in 'first operand'; found '['").

In NASM or YASM, BYTE [ecx] accesses memory at address ecx. BYTE PTR [ecx] is a syntax error ("error: comma, colon or end of line expected" in NASM, "undefined symbol `PTR'" in YASM).

In TASM, BYTE PTR [ecx] and BYTE [ecx] are equivalent--both access memory at address ecx.

However, in the Gnu assembler gas, when using the intel syntax BYTE PTR [ecx] accesses memory at ecx, but BYTE [ecx] actually accesses memory at address ecx+1. That is, BYTE [ecx] is equivalent to BYTE PTR [ecx+1], which does not appear to be either sane or documented.

Gnu assembler version 2.18, 2.24, or 2.26.1:

cat > foo.S << EOF
.intel_syntax noprefix
 movb BYTE [ecx], 0 
 movb BYTE PTR [ecx], 0 
.att_syntax prefix
EOF

as foo.S
objdump -dM intel a.out

0:  67 c6 41 01 00          mov    BYTE PTR [ecx+0x1],0x0
5:  67 c6 01 00             mov    BYTE PTR [ecx],0x0