汇编中的js和jb指令

Question

I am having a hard time understanding what exactly js and jb instruction do. I understand that jb is jump if below. But, what would be the difference between jb and jle . And similarly, js seems to me that it is equivalent to jb , as it means jump if signed. Any help would be appreciated.

Answer 1

jb (and ja ) branch based on the unsigned result of the flags, as opposed to the signed branch condition for jg , jge , jl , and jle .

In an unsigned comparison, the MSB is included as part of the number itself and not an indication of its sign. For example:

 ; Intel                          ; ; AT&T
 mov eax, 08000000h               ; mov $0x8000000, %eax
 mov ecx, 00000001h               ; mov $0x0000001, %ecx
 cmp eax, ecx                     ; cmp %ecx, %eax
 jl mybranch ; branch taken       ; jl mybranch ; branch taken

Whereas:

 mov eax, 08000000h               ; mov $0x8000000, %eax
 mov ecx, 00000001h               ; mov $0x0000001, %ecx
 cmp eax, ecx                     ; cmp %ecx, %eax
 jb mybranch ; branch not taken   ; jb mybranch ; branch not taken

js will branch based solely on the state of the sign flag in the (R|E)FLAGS register

Answer 2

There exists a handy table that does explain very well which Jcc instruction to use:

Jump conditions and flags:

Mnemonic        Condition tested  Description  
jo              OF = 1            overflow 
jno             OF = 0            not overflow 
jc, jb, jnae    CF = 1            carry / below / not above nor equal
jnc, jae, jnb   CF = 0            not carry / above or equal / not below
je, jz          ZF = 1            equal / zero
jne, jnz        ZF = 0            not equal / not zero
jbe, jna        CF or ZF = 1      below or equal / not above
ja, jnbe        CF or ZF = 0      above / not below or equal
js              SF = 1            sign 
jns             SF = 0            not sign 
jp, jpe         PF = 1            parity / parity even 
jnp, jpo        PF = 0            not parity / parity odd 
jl, jnge        SF xor OF = 1     less / not greater nor equal
jge, jnl        SF xor OF = 0     greater or equal / not less
jle, jng    (SF xor OF) or ZF = 1 less or equal / not greater
jg, jnle    (SF xor OF) or ZF = 0 greater / not less nor equal