Michael S (already5chosen.delete@this.yahoo.com) on February 27, 2013 4:24 pm wrote:
> I say 12 rather than 14, because I don't think that negative/non-negative conditions are really fundamental.

The sign flag helps some times and is essentially free.

> Now, do we really need 4 flag bits in order to implement 12 (or even all 14) fundamental
> conditions? No, we don't. Up to 64 conditions, including all 14 defined in x86/ARM?SPARC,
> can be easyly derived from just 7 distinct states = 3 bits.
> For example, like those:
> 0 - unsigned_less && signed_less. Implied non-overflow && negative
> 1 - unsigned_less && signed_greater && non-overflow. Implied non-negative
> 2 - unsigned_less && signed_greater && overflow. Implied negative
> 3 - unsigned_greater && signed_less && non-overflow. Implied negative
> 4 - unsigned_greater && signed_less && overflow. Implied non-negative
> 5 - unsigned_greater && signed_greater. Implied non-overflow && non-negative
> 6 - equal. Implied non-overflow && non-negative

Encoding in three bits have the same "advantage" of killing the sign flag at the expense of more complex (and time consuming) flag calculation, but the sign flag is free, so no real gain, also... 3 bitsT allow up to 8 resulting states, but there are 9, no?

mov eax, 1
add eax, 1

Non-Zero
Non-Negative
Non-Carry
Non-Overflow

mov eax, 0
add eax, 0

Zero
Non-Negative
Non-Carry
Non-Overflow

mov eax, -1
add eax, 2

Non-Zero
Non-Negative
Carry
Non-Overflow

mov eax, -1
add eax, 1

Zero
Non-Negative
Carry
Non-Overflow

mov eax, 80000000h
add eax, 80000000h

Zero
Non-Negative
Carry
Overflow

mov eax, 80000001h
add eax, 80000000h

Non-Zero
Non-Negative
Carry
Overflow

mov eax, -2
add eax, 1

Non-Zero
Negative
Non-Carry
Non-Overflow

mov eax, 70000000h
add eax, 70000000h

Non-Zero
Negative
Non-Carry
Overflow

mov eax, -1
add eax, -1

Non-Zero
Negative
Carry
Non-Overflow