Assuming the two values I want to compare are the only values on the floating point stack, then the fcomi instruction will compare the values and set the CPU-flags so that the jejnejlMeanwhile on pg. In other words it's testing the sign, overflow and zero flags, but FCOMI doesn't set sign or overflow!
Depending on which conditions you wish to jump, you should look at the possible comparison results and decide when you want to jump.
In other words the condition codes match those for using unsigned comparisons. The same goes if you're using FMOVcc.
If you care what your code does with NaNs, you may need an extra jp or jnp. If you want the unordered case to take the same execution path as below or equal, then ja is all you need. Here you should use JA if you want it to print ie. Note this might be a little confusing due to the fact that we only print if we don't jump.
Regarding your second question: by default fcomi doesn't pop anything. You should always clear the fpu register stack after usage, so all in all your program ends up like this assuming you want the message printed:. Learn more. Asked 8 years, 8 months ago. Active 8 months ago. Viewed 14k times. I have tried to find resources on how to do this online and from what I understand it works like this: Assuming the two values I want to compare are the only values on the floating point stack, then the fcomi instruction will compare the values and set the CPU-flags so that the jejnejlI'm asking because this only works sometimes.
For example:. What am I doing wrong? PS: does the fcomip pop only one value or does it pop both? Active Oldest Votes. Peter Cordes k 29 29 gold badges silver badges bronze badges. Very impressive answer. One minor comment: the opposite of ja is jbenot jb. Ray Toal: You are absolutely right. Even though it won't make any difference in this case I changed the example since it makes more sense that way. Sign up or log in Sign up using Google. Sign up using Facebook. Sign up using Email and Password.
Post as a guest Name.Getting the sense for jumps and flags has long been a troublesome area for me, especially since the Intel assembler book shows 32 of these, all with similar-sounding names.
Looking more closely I found that many of the instructions were synonyms for each other, and in practice the whole gamut is not needed, and in the process found that my copy of Intel's Programmer's Reference Manual gave an incorrect description for one of the instructions. The x86 processors have a large set of flags that represent the state of the processor, and the conditional jump instructions can key off of them in combination.
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Condition Codes 1: Condition Flags and Codes
So I have grouped these functionally, with all instruction synonyms in the same row. CF - carry flag Set on high-order bit carry or borrow; cleared otherwise PF - parity flag Set if low-order eight bits of result contain an even number of "1" bits; cleared otherwise ZF - zero flags Set if result is zero; cleared otherwise SF - sign flag Set equal to high-order bit of result 0 if positive 1 if negative OF - overflow flag Set if result is too large a positive number or too small a negative number excluding sign bit to fit in destination operand; cleared otherwise More Tech Tips.
Jump if below Jump if not above or equal Jump if carry. Jump if not below Jump if above or equal Jump if not carry.A status registerflag registeror condition code register CCR is a collection of status flag bits for a processor.
The status register is a hardware register that contains information about the state of the processor. The status register lets an instruction take action contingent on the outcome of a previous instruction. Typically, flags in the status register are modified as effects of arithmetic and bit manipulation operations.
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For example, a Z bit may be set if the result of the operation is zero and cleared if it is nonzero. Other classes of instructions may also modify the flags to indicate status.
The flags are read by a subsequent conditional instruction so that the specified action depending on the processor, a jump, call, return, or so on occurs only if the flags indicate a specified result of the earlier instruction. Such machines either do not pass implicit status information between instructions at all, or they pass it in an explicitly selected general purpose register.
A status register may often have other fields as well, such as more specialized flags, interrupt enable bits, and similar types of information. During an interrupt, the status of the thread currently executing can be preserved and later recalled by storing the current value of the status register along with the program counter and other active registers into the machine stack or some other reserved area of memory.
This is a list of the most common CPU status register flags, implemented in almost all modern processors. Status flags enable an instruction to act based on the result of a previous instruction. In pipelined processors, such as superscalar and speculative processors, this can create hazards that slow processing or require extra hardware to work around them. Some very long instruction word processors dispense with the status flags. A single instruction both performs a test and indicates on which outcome of that test to take an action, such as Compare a with b and Jump to c if Equal.
The result of the test is not saved for subsequent instructions. Another alternative to the status register is for processor instructions to deposit status information in a general-purpose register when the program requests it. Conditional branches act based on the value in the general-purpose register.
To test for other conditions, a program uses an equivalence formula. For example, MIPS has no "carry bit" but a program performing multiple-word addition can test whether a single-word addition of registers overflowed by testing whether the sum is lower than an operand: . The sltu instruction sets tmp to 1 or 0 based on the specified comparison of its two other operands. Here, the general-purpose register tmp is not used as a status register to govern a conditional jump; rather, the possible value of 1, indicating carry from the low-order addition, is added to the high-order word.
X86 Assembly/Control Flow
Fortunately, those two carries may be added to each other without risk of overflow, so the situation stabilizes at five instructions per word added. From Wikipedia, the free encyclopedia. Redirected from Condition Code Register.
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Indicates that the signed result of an operation is too large to fit in the register width using two's complement representation. Half-carry flag Auxiliary flag Digit Carry Decimal adjust flag. Indicates that a bit carry was produced between the nibbles typically between the 4-bit halves of a byte operand as a result of the last arithmetic operation. Such a flag is generally useful for implementing BCD arithmetic operations on binary hardware.But no matter the name, the register is not directly accessible except for a couple of instructions - see below.
Instead, individual flags are referenced in certain instructions, such as conditional Jump or conditional Set, known as Jcc and SETcc where cc means "condition code" and references the following table:. In 16 bits, subtracting 1 from 0 is either 65, or -1 depending on whether unsigned or signed arithmetic is used - but the destination holds 0xFFFF either way. It's only by interpreting the condition codes that the meaning is clear.
It's even more telling if 1 is subtracted from 0x : in unsigned arithmetic, that merely changes 32, into 32, ; while in signed arithmetic it changesinto 32, - a much more noteworthy overflow! The condition codes are grouped into three blocks in the table: sign-irrelevant, unsigned, and signed. The naming inside the latter two blocks uses "Above" and "Below" for unsigned, and "Greater" or "Less" for signed. The above condition codes are useful for interpreting predefined concepts, but the actual flag bits are also available directly with the following two instructions:.
Only certain flags are copied across with these instructions. As the Intel architecture improved, it got faster through such technology as caches and super-scalar execution. That had to optimise access to the system by making assumptions. To control those assumptions, more flags were needed:.
Condition Codes But no matter the name, the register is not directly accessible except for a couple of instructions - see below. This is set with the STI instruction to globally enable interrupts, and cleared with the CLI instruction to globally disable interrupts.
DF The Direction Flag. Memory-to-memory operations such as CMPS and MOVS to compare and move between memory locations automatically increment or decrement the index registers as part of the instruction.
The DF flag dictates which one happens: if cleared with the CLD instruction, they're incremented; if set with the STD instruction, they're decremented. TF The Trap Flag. This is a debug flag. Setting it will put the processor into "single-step" mode: after each instruction is executed it will call the "Single Step Interrupt Handler", which is expected to be handled by a debugger.Every practical general-purpose computing architecture has a mechanism of conditionally executing some code.
Such mechanisms are used to implement the if construct in C, for example, in addition to several other cases that are less obvious. Arm, like many other architectures, implements conditional execution using a set of flags which store state information about a previous operation.
I intend, in this post, to shed some light on the operation of these flags. Of course, the Architecture Reference Manual is the definitive source of information, so if you need to know about a specific corner-case that I do not cover here, that is where you need to look.
The last two instructions are of particular interest. The cmp compare instruction compares r4 with 0, and the bne instruction is simply a b branch instruction that executes if the result of the cmp instruction was "not equal". The code works because cmp sets some global flags indicating various properties of the operation.
The bne instruction — which is really just a b branch with a ne condition code suffix — reads these flags to determine whether or not to branch 1. Adding the s suffix to sub causes it to update the flags itself, based on the result of the operation. This suffix can be added to many but not all arithmetic and logical operations 2. In the rest of the article, I will explain what the condition flags are, where they are stored, and how to test them using condition codes.
If you have an Arm platform or emulator handy, the attached ccdemo application can be used to experiment with the operations discussed in the article. The application allows you to pick an operation and two operands, and shows the resulting flags and a list of which condition codes will match.
When writing assembly code, it can also be a rather useful development tool. The simplest way to set the condition flags is to use a comparison operation, such as cmp. This mechanism is common to many processor architectures, and the semantics if not the details of cmp will likely be familiar. In addition, we have already seen that many instructions such as sub in the example can be modified to update the condition flags based on the result by adding an s suffix.
That's all well and good, but what information is stored, and how can we access it? The flags indicate simple properties such as whether or not the result was negative, and are used in various combinations to detect higher-level relationships such as "greater than" and suchlike.
Once I have described the flags, I will explain how they map onto condition codes such as ne in the previous example. The N flag is set by an instruction if the result is negative. In practice, N is set to the two's complement sign bit of the result bit The C flag is set if the result of an unsigned operation overflows the bit result register.
This bit can be used to implement bit unsigned arithmetic, for example. The V flag works the same as the C flag, but for signed operations. The result of the operation would be 0xbut the top bit is lost because it does not fit into the bit destination register and so the real result is 0x In this case, the flags will be set as follows:.
If you fancy it, you can check this with the ccdemo application. The output looks like this:. We have worked out how to set the flags, but how does that result in the ability to conditionally execute some code?
Being able to set the flags is pointless if you cannot then react to them. The most common method of testing the flags is to use conditional execution codes. In effect, attaching one of the condition codes to an instruction causes it to execute if the condition is true. Otherwise, it does nothing, and is essentially a nop. The following table lists the available condition codes, their meanings where the flags were set by a cmp or subs instructionand the flags that are tested:.
It is fairly obvious how the first few work because they test individual flags, but the others rely on specific combinations of flags. In practice, you very rarely need to know exactly what is happening; the mnemonics hide the complexity of the comparisons. The cmp instruction that we saw in the first example can be thought of as a sub instruction that doesn't store its result: if the two operands are equal, the result of the subtraction will be zero, hence the mapping between eq and the Z flag.
Of course, we could just use a sub instruction with a dummy register, but you can only do that if you have a register to spare.Contents: Registers Memory and Addressing Instructions Calling Convention This guide describes the basics of bit x86 assembly language programming, covering a small but useful subset of the available instructions and assembler directives. There are several different assembly languages for generating x86 machine code.
MASM uses the standard Intel syntax for writing x86 assembly code. The full x86 instruction set is large and complex Intel's x86 instruction set manuals comprise over pagesand we do not cover it all in this guide. For example, there is a bit subset of the x86 instruction set. Using the bit programming model can be quite complex. It has a segmented memory model, more restrictions on register usage, and so on.
In this guide, we will limit our attention to more modern aspects of x86 programming, and delve into the instruction set only in enough detail to get a basic feel for x86 programming. Modern i. The register names are mostly historical. For example, EAX used to be called the accumulator since it was used by a number of arithmetic operations, and ECX was known as the counter since it was used to hold a loop index.
Whereas most of the registers have lost their special purposes in the modern instruction set, by convention, two are reserved for special purposes — the stack pointer ESP and the base pointer EBP. The least significant byte of AX can be used as a single 8-bit register called ALwhile the most significant byte of AX can be used as a single 8-bit register called AH. These names refer to the same physical register. These sub-registers are mainly hold-overs from older, bit versions of the instruction set.
However, they are sometimes convenient when dealing with data that are smaller than bits e. When referring to registers in assembly language, the names are not case-sensitive.
x86 Instruction Set Reference
For example, the names EAX and eax refer to the same register. Example declarations:. Unlike in high level languages where arrays can have many dimensions and are accessed by indices, arrays in x86 assembly language are simply a number of cells located contiguously in memory. An array can be declared by just listing the values, as in the first example below. Two other common methods used for declaring arrays of data are the DUP directive and the use of string literals.Wikipedia has related information at Control flow.
Almost all programming languages have the ability to change the order in which statements are evaluated, and assembly is no exception. The instruction pointer EIP register contains the address of the next instruction to be executed. To change the flow of control, the programmer must be able to modify the value of EIP. This is where control flow functions come in. Performs a bit-wise logical and on arg0 and arg1 the result of which we will refer to as commonBits and sets the ZF zeroSF sign and PF parity flags based on commonBits.
CommonBits is then discarded. Performs a comparison operation between minuend and subtrahend. The comparison is performed by a signed subtraction of subtrahend from minuendthe results of which can be called difference. Difference is then discarded.
If subtrahend is an immediate value it will be sign extended to the length of minuend. The jump instructions allow the programmer to indirectly set the value of the EIP register. The location passed as the argument is usually a label.Assembly Language Tutorial 4: Flags Register CF, OF, ZF ,AF, SF, PF
The first instruction executed after the jump is the instruction immediately following the label. All of the jump instructions, with the exception of jmpare conditional jumpsmeaning that program flow is diverted only if a condition is true.
These instructions are often used after a comparison instruction see abovebut since many other instructions set flags, this order is not required. Loads EIP with the specified address i.
Loads EIP with the specified address, if operands of previous cmp instruction are equal. For example:. Loads EIP with the specified address, if operands of previous cmp instruction are not equal.
Loads EIP with the specified address, if the minuend of the previous cmp instruction is greater than the second performs signed comparison. Loads EIP with the specified address, if the minuend of the of previous cmp instruction is greater than or equal to the subtrahend performs signed comparison. Loads EIP with the specified address, if the minuend of the previous cmp instruction is greater than the subtrahend. That means, the following piece of code always jumps unless rbx is -1toobecause negative one is represented as all bits set in the two's complement.