What values can the carry flag hold? Is it just 0x00 and 0x01 (boolean) or is it 16 (or 32/64) bits like the rest of the CPU registers?
How do I check its status? Do I just use it like a normal CPU register like cmp cf, 0x00
then jg <jump destination>
?
I am writing a mini-OS. Is it good practice to use it for my own purposes, or should it be reserved for exclusive write-permissions for the CPU, and all I do is read from it?
1. The carry flag is set if the addition of two numbers causes a carry out of the most significant (leftmost) bits added. 2. The carry (borrow) flag is also set if the subtraction of two numbers requires a borrow into the most significant (leftmost) bits subtracted.
In computer processors the carry flag (usually indicated as the C flag) is a single bit in a system status register/flag register used to indicate when an arithmetic carry or borrow has been generated out of the most significant arithmetic logic unit (ALU) bit position.
To conditionally branch on the status of the carry flag (CF), you would use JC or JNC . JC will branch if the carry flag is set (CF == 1), whereas JNC will branch if the carry flag is not set (CF == 0). The mnemonics for these opcodes are simply "Jump if Carry" and "Jump if Not Carry".
The status flags reflect the outcomes of arithmetic and logical operations performed by the CPU. • The carry flag (CF) is set when the result of an unsigned arithmetic operation is too large to fit into the destination.
It's a flag, it can only hold true or false (technically 1 or 0, but effectively the truth values as shown).
In terms of using it, no, you don't compare it to something and then use jg
. It's at the same level of abstraction as other flags so you can just use:
jc somewhere ; jump if carry flag is set
jnc somewhere_else ; jump if carry flag is not set
It's set automatically by certain instructions so, for example, to add two values and detect carry, you can use something like:
add ax,bx
jc too_big
And, while it's mostly set by those instructions, you can also do it manually with stc
(set), clc
(clear) and cmc
(complement). For example, it's often useful to clear it before-hand if you're entering a loop where the value is carried forward to the next iteration.
there was this little book that once came with borland turbo assembler that listed all x86 instructions, together with their number of cpu cycles and flags affected for every model of processor individually... i suggest you go find one of those books and read it... 2: no, you cannot use cmp etc directly on the flags REGISTER as it's not memory but a register in the cpu, you can however use a couple of designated results or move the whole thing first onto the stack and then into ram or the other way around with the instructions below
CLC (clear (0) carry bit) STC (set carry flag to 1) JC (branch if carry set) JNC (branch if carry not set) PUSHF (pop flags onto stack) POPF (move latest entry on stack into flags register)
if i recall correctly... there probably are some more ways of doing things.
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