push 0
    

  mov eax,0  ; or should i use "xor eax,eax" 
    

There's a short paragraph about the size settings

Immediate value as an operand for push instruction without a size operator is
by default treated as a word value if assembler is in 16-bit mode and as a
double word value if assembler is in 32-bit mode, shorter 8-bit form of this
instruction is used if possible, word or dword size operator forces the push
instruction to be generated in longer form for specified size. pushw and pushd
mnemonics force assembler to generate 16-bit or 32-bit code without forcing it
to use the longer form of instruction.
    

force assembler to generate 16-bit or 32-bit code

Is it useful at all to generate missmatching code (16-bit when other code is
32-bit and CPU is in 32-bit PM) or 32-bit when other code is 16-bit ?

The text does NOT mention the amount of pushed data ... and also
does not say anything on pushing 1 byte only

use16
;not allowed
push/pop      -2^31...-1-2^15 | 2^15..-1+2^31
pushw/popw    -2^31...-1-2^15 | 2^15..-1+2^31
;BUT allowed pushes/pops dword onto/from stack, 4 bytes in machine code
pushd/popd    -2^31...-1-2^15 | 2^15..-1+2^31

;pushes/pops word onto/from stack, 2 bytes in machine code
push/pop      -2^15...-1-2^7 | 2^7..-1+2^15
pushw/popw    -2^15...-1-2^7 | 2^7..-1+2^15
;BUT this pushes/pops dword onto/from stack, 4 bytes in machine code
pushd/popd    -2^15...-1-2^7 | 2^7..-1+2^15

;pushes/pops word onto/from stack, 1 bytes in machine code
push/pop    -2^7..-1+2^7
;BUT this pushes/pops word onto/from stack, 2 bytes in machine code
pushw/popw    -2^7..-1+2^7
;BUT this pushes/pops dword onto/from stack, 4 bytes in machine code
pushd/popd    -2^7..-1+2^7



use32
;pushes/pops dword onto/from stack, 4 bytes in machine code
push/pop      -2^31...-1-2^15 | 2^15..-1+2^31
pushd/popd    -2^31...-1-2^15 | 2^15..-1+2^31
;BUT not allowed
pushw/popw    -2^31...-1-2^15 | 2^15..-1+2^31

;pushes/pops dword onto/from stack, 4 bytes in machine code
push/pop      -2^15...-1-2^7 | 2^7..-1+2^15
pushd/popd    -2^15...-1-2^7 | 2^7..-1+2^15
;BUT this pushes/pops word onto/from stack, 2 bytes in machine code
pushw/popw    -2^15...-1-2^7 | 2^7..-1+2^15

;pushes/pops word onto/from stack, 1 bytes in machine code
push/pop    -2^7..-1+2^7
;BUT this pushes/pops dword onto/from stack, 4 bytes in machine code
pushd/popd    -2^7..-1+2^7
;BUT this pushes/pops word onto/from stack, 2 bytes in machine code
pushw/popw    -2^7..-1+2^7
//sorry, no use64 yet
    

The text does NOT mention the amount of pushed data ... and also does not say anything on pushing 1 byte only

Or to make it even more clear: "the shorte 8bit form" is only usable for values from -128 to 127 (...)

No byte push is possible.

Also I highly disrecommend 1 byte pushes/pops

Well, if you code stuff like DOS extenders (flat/unreal mode) where you switch from 16bit to 32bit mode for example

The size of the operand is the amount of pushed data aswell

Immediate value as an operand for push instruction without a 
size operator results by default into pushing 16 bits if assembler
is in 16-bit mode and 32 bits if assembler is in 32-bit mode, 
shorter 8-bit form of this instruction is used if possible, word or 
dword size operator forces the push instruction to be generated 
in longer form for specified size and also push this size. pushw 
and pushd mnemonics force assembler to generate instructions 
pushing 16-bit or 32-bit without forcing it to use the longer form 
of instruction. Pushing 1 byte only is not possible.
    

OK, but this operand is in source only and its size in NOT the size in output code

This is a bit tricky.

In 16-bit mode "push 65408" will also generate the short form

you usually focus on what the instruction does, not how it is encoded.

but not always, see XOR EAX,EAX issue

MCD wrote:

Or to make it even more clear: "the shorte 8bit form" is only usable for values from -128 to 127 (...)

This is a bit tricky. In 16-bit mode "push 65408" will also generate the short form.

I guess there is an analogous case with "push 4294967168" in 32bit mode

mov   BYTE [esp-1], 0 ;used esp-1 to avoid address interlock
dec   esp, 1    

sub   al, al
dec   esp, 1
mov  BYTE [esp], al    

mov   BYTE [esp-1], 0
dec   esp
sub   al, al    

mov   al, BYTE [esp]
inc   esp    

loose two cycles do to address misalignment and an address interlock.

mov   eax, [edx]
add   eax, 4
inc   ecx    

mov   eax, [edx]
inc   ecx
add   eax, 4    

sub   eax, eax    

xor   eax, eax    

Also XOR is preferred over SUB because of the special hardware support within the cpu to break dependency chains, SUB reg,reg will wait for any previous results to be written before doing the SUB but XOR reg,reg is smart and won't wait.

The Pentium 4 processor provides special support for XOR, SUB, and PXOR opera-
tions when executed within the same register. This recognizes that clearing a register
does not depend on the old value of the register. The XORPS and XORPD instructions
do not have this special support. They cannot be used to break dependence chains.
In Intel Core Solo and Intel Core Duo processors, the XOR, SUB, XORPS, or PXOR
instructions can be used to clear execution dependencies on the zero evaluation of
the destination register.