Imul x86 example The following examples show how a 8-bit number in AL is converted to a 16-bit number in AX: Unsigned: mov al, [bNum1] mov ah, 0 Signed: IMUL - 8086. Sep 25, 2012 · Second, you give an example of using imul with an immediate operand, another thing that's unavailable for DIV/IDIV. This form is identical to that used by the MUL instruction. /a. This instruction has three forms, depending on the number of operands. The reader is prompted with examples to predict However turn over to IMUL and you'll see there are many forms of it that accept an immediate. Multiplication of Signed Numbers. when operand is a word : (DX AX) = AX * operand. out 2 4 -" = -2; ". ISA is the only aspect of x86 that did not change x86-64 compilers mostly use the RISC-like instructions The internals of modern x86 CPUs are RISC-like x86 ISA attributes Variable-length byte-granularity instructions Most instructions overwrite one operand Two-address instead of three-address style Most instructions allow one operand in memory Oct 29, 2017 · As said by Fifoernik, if you want to multiply a 16 bit register for a constant, you may need up to 32 bit result (furthermore you can have negative input-number); all the routines I've written are successfully tested and they are about three time more fast than IMUL (8086 assembly guides of Peter Norton): Example A; multiply AX for 41: IMUL BX, 12h IMUL DX, 1200h IMUL CX, DX, 12h IMUL BX, SI, 1200h IMUL DI, word ptr [BX + SI], 12h IMUL SI, word ptr [BP-4], 1200h Note that since the lower half is the same for unsigned and signed multiplication, this version of the instruction can be used for unsigned multiplication as well. text global _start extern multiply _start: mov rcx, 5 ; a = 5 mov rdx, 10 ; b = 10 call multiply ; result is in rax call ExitProcess multiply: imul rax, rcx, rdx ret Learning Strategies. For example, if you add 1 to the AL register that contains 255, you will get AL=0, Carry = 1. Indirect mode: the operand is the value stored in a memory location, which is specified by an offset from the value in a register. This example waits for the user to input data, outputs it, then fills the buffer with Z’s and outputs it again. Koether (Hampden-Sydney Dec 11, 2015 · x86 Architecture. 1. imul esi, edi, 25 ; ESI ← EDI * 25 Division idiv not - bitvise logical not (flips all bits) neg - negation neg eax ; EAX ← - EAX Example Background. When multiplying N-bit number by other N-bit one, the low N bits of the result are the same no matter whether you do the signed or unsigned multiplication - you can read first chapter of my tutorial, which I have written to explain such things (though I'm not sure whether I wrote it clearly enough there, if you have any suggestions or questions, please let me know). sub src, dest Subtract src from dest. The IMUL instruction takes one, two or three operands. Mar 6, 2017 · x86 imul's two forms. e 386 and beyond) x86 processor have eight 32-bit general purpose registers, as decipted in following Figure. As you typically have 32 or 64-bit input values for signed division, you often need to use CDQ or CQO to sign-extend EAX into EDX or RAX into RDX just before the division . (This is true across all mainstream and low-power Intel/AMD that are still rele Nov 4, 2016 · I'm using IMUL (signed multiply) as it's a tiny fraction faster when used in r32,r32 form (discarding upper 32b of result, so edx is preserved too), but the algorithm is designed to work only with non-negative [n,k] (like P(0, 0) = 1). This tells the assembler to only move one byte worth of data. So a separate multi-operand mul isn't needed. It's possible but unlikely that there's some condition where that CPUs sets ZF=1. For example, X86-imul-op/en-rm. Nov 22, 2020 · thanks to twos complement addition and subtraction dont need to know nor care to perform the operation. Below is our overview of its features that are relevant to CS107. I think Intel picked imul as the one that would have multiple explicit operands so that imul r32, r32, sign-extended-imm8 would make more sense, because sign-extension is probably more useful than zero-extension. Jan 8, 2020 · I have this code: mov rax, 0x93f3ffc2fbc7a1ce mov rbx, 0x5862d8a05a385cbe imul eax, ebx How does imul work for 64-bit assembly? Will the overflow aex be written in the first 32 bits of rax? Jul 14, 2016 · Description. The three forms of the IMUL instruction are similar in that the length of the product is calculated to twice the length of the operands. MUL/IMUL [r/m32] IMUL [reg], [r/m32] IMUL [reg], [r/m32], [immediate] Performs a signed multiplication of two operands. imul S, D D ← D * S •If you specify one operand, it multiplies that by %rax, and splits the product section . The content of the registers ebx and edx is destroyed:. Mar 18, 2019 · The three forms of the IMUL instruction are similar in that the length of the product is calculated to twice the length of the operands. and control-flow. Apr 15, 2018 · The following code will multiply the contents of the registers ecx and edx and store the result in register eax. This test demonstrates the use of IMUL (signed multiplication). There's no immediate mul, but there is non-widening imul-immediate in 186 and newer, and imul reg, r/m in 386 and newer. Recall that every write to a 32-bit register will zero the upper half of the corresponding 64-bit register, so the effect is the same as long as your constant is unsigned 32 bits, and the encoding of mov $7, %edi is one byte shorter as Multiplication imul imul eax, [var] ; multiply the contents of EAX by the ; 32-bit contents of the memory location ; var. There are two instructions for multiplying binary data. You might be right I just can't find any resource to back up that the imul instruction knows that the resulting product is either positive so it sign extends 0 or negative so it sign extends 1. Direct Addressing. In this section, we will look at important examples of x86 instructions from each category. Of course for division by constants idiv and div (and aam) are very inefficient. Examples dec eax — subtract one from the contents of EAX. mov ebx, 1 mov eax, 0 repeat: test ecx, ebx jz dontadd add eax, edx dontadd: add edx, edx add ebx, ebx jnz repeat Dec 23, 2017 · Modern (i. imul — Integer multiplication The imul instruction has two basic formats: two-operand (first two syntax listings above) and three-operand (last two syntax listings above). One relevant example is that on x86, the flags register is always clobbered. Quotient in EAX, remainder in EDX Jun 1, 2023 · But imul bl only widens its result to 16-bit, not all the way to a convenient 32-bit, so you'd more often want to have your inputs widened to 32-bit for imul ecx, ebx or something, since 32-bit is the most efficient operand-size most of the time on x86-64. There is one obscure instruction which is actually an immediate-div, doing 8 bit / imm8 => 8bit quotient/remainder, rather than 16 / 8 => 8. Also What's 'new' in a 'new' processor when viewed from programmer's point uses imul as an example. See also our x86-64 sheet for a compact reference. To get the product of a register and a constant and store it in another register, the naïve way is to do this: imul ecx, 3 ; Set ecx to 5 times its previous value imul edx, eax, 5 ; Store 5 times the contend of eax in edx Use lea. x86-64 machine code is the native language of the processors in most desktop and laptop computers. It is set when you have a carry after an arithmetic operation. text _start: mov al, 2 imul BYTE [byteVariable] You can replace imul with mul, and the result would still be the same (-10). Oct 18, 2010 · In these cases, the low 16 or low 32 bits of the result is the same for imul as they would be for an equivalent mul, only the flags (CF and OF) may differ. The point of CDQ is to set up EDX prior to a division by a 32-bit operand, since the dividend is EDX:EAX. Syntax IMUL r/m32 EDX:EAX = EAX * r/m doubleword IMUL r32,r/m32 Jun 16, 2021 · mul vs. Registers Sep 24, 2016 · Same place as for any other x86 instruction: in Intel® 64 and IA-32 Architectures Software Developer’s Manual – Michael. We use the IMUL instruction. This is because mul and its sibling imul produce an output twice as wide as their operands 1. 8086 Singed Multiplication Instruction (IMUL) The IMUL instruction allows the multiplication of two signed operands. Therefore, you will be doing division as a repeated subtraction and multiplication as a repeated addition. loop xor edi,edi mov eax,231 syscall ; sys_exit_group(0) built (with nasm + ld) into a static executable on x86-64 Linux, this runs (on Skylake) in the expected 750M clock cycles for each chain of 25 * 10M imul instructions times 3 cycle latency. See Why does integer division by -1 (negative one) result in FPE? for an example of x86 vs. In your example, multiplying by al produces an output in ax and no overflow is generated. In x86_64 compilers use multi-operand imul to produce 64-bit results even when the inputs were only 32-bits because they still typically have better latency Nov 19, 2015 · For example for implementing signed 128-bit multiplication with SSE2/AVX2/AVX512 or for implementing 256-bit multiplication when the instruction set only does 128-bit multiplication (such as with x86-64). The other four registers can be accessed as 32-bit or 16-bit. ) CS107 x86-64 Reference Sheet Common instructions mov src, dst dst = src movsbl src, dst byte to int, sign-extend movzbl src, dst byte to int, zero-fill cmov src, reg reg = src when condition holds, using same condition suffixes as jmp lea addr, dst dst = addr add src, dst dst += src sub src, dst dst -= src imul src, dst dst *= src coder32 edition of X86 Opcode and Instruction Reference. MUL/IMUL multiplier Apr 18, 2019 · The three forms of the IMUL instruction are similar in that the length of the product is calculated to twice the length of the operands. In direct addressing, the operand's address is directly specified in the instruction itself. imul vs. Unfamiliar syntax for the "imul" instruction in x86 assembly. May 27, 2022 · With divisor size 4, this means that EDX are the bits 32-63 and EAX are bits 0-31 of the input number (with lower bit numbers being less significant, in this example). There is more extensive coverage on these topics in Chapter 3 of the B&O textbook. An example of the bits in the FLAGS set by the program is the Direction Flag. imul src, dest Multiply dest by src. There are multiple flavours of Intel syntax, the major ones being NASM (mov dword [symbol_name], 1) and MASM (including GAS's . Split the 64bit quantities into two 32bit words and multiply the low words to the lowest and 2nd lowest word of the result, then both pairs of low and high word from different numbers (they go to the 2nd and 3rd lowest word of the result) and finally both high words into the 2 highest words of the result. l1 mov eax,edx . Nov 28, 2015 · Examples idiv ebx ; EDX:EAX / EBX. Unfortunately, this only works with the AX register. Mar 12, 2018 · Using movsxd before a 64-bit imul with 2 explicit operands is better on most CPUs, but on a few CPUs where 64-bit imul is slow (AMD Bulldozer-family, or Intel Atom), you could use. If only 1 register provided, multiplies it by eax. DC /1: FMUL m64fp Jan 31, 2020 · For signed numbers the x86 CPUs have instructions named cbw (8 to 16 bits) and cwd (16 to 32 bits). Irvine. The operation of MUL and IMUL instructions are same. AsmDB comes with a tool called x86util. In this case, you're looking for the MUL and DIV instructions (in case you're operating on unsigned operands) or IMUL and IDIV instructions, which are used for signed integer multiplication and division. The x86 Architecture Robb T. The register names are mostly hsitorical. Algorithm: when operand is a byte : AX = AL * operand. The first operand (second source) can be a register or memory. The MUL (Multiply) instruction handles unsigned data and the IMUL (Integer Multiply) handles signed data. It's faster to use a combination of shifts and adds. g. imul is also 1 uop, but mov+shl+add is 3 for the front-end. Register Addressing. The x86 has a special short encoding for the 4 shift instructions that can be used with no 2nd operand or you can encode the value 1 as a 2nd operand which is a longer instruction that does the same thing. For example movl %eax, 8(%rsp) This moves the 32 bits stored in %eax (the lower half of %rax) Jun 20, 2016 · That's right, modern x86 CPUs (especially Intel) have very high performance multipliers. Keep in mind, though, that this is only for convenience. imul esi, 24 / mov eax, [rdi + rax + 12]. Example: MOV AX, BX - moves the contents of register BX into register AX. edu/wies CSCI-UA. BigMul implementations (currently using intrinsics example movq %rsp, %rax which puts the stack pointer into register %rax. There is no program that appears in this text that could not be recoded using only 8088 assembly language instructions. The translation would be I was trying to do this arithmetic operation in assembly but i was getting the wrong answer every time, not sure how exactly I'm suppose to do it These questions are from a textbook, and i do have x86-64 (also known as just x64 and/or AMD64) is the 64-bit version of the x86/IA32 instruction set. In register addressing, the operand is placed in one of the CPU registers. You also can't change Computer Systems Organization Thomas Wies wies@cs. Presumably the designers went with the mnemonic imul because the forms with an 8 bit immediate do sign-extend that immediate. But if you had EAX = 0x80000000 you'd get EDX = 0xFFFFFFFF since the most significant bit of EAX is set. In this case, the problem is that the : after table (table:) makes label of type "code" that is normally used as a branch or call target, so jmp [table] or jmp table, branches to table as if it were code. ) The compiler will not emit a diagnostic if you explicitly clobber a register that is also implicitly clobbered. The product of two 32 bit values doesn't necessarily fit in 32 bits: the full multiply result can take up to 64 bits. edu IMUL Examples The following fragment computes 8-bit signed multiplication (48 × 4) : mov al, 48 mov bl, 4 imul bl ; AX = 00C0h (decimal +192), OF = 1 May 24, 2014 · The MUL/IMUL instructions. This would be very useful for Math. Nov 18, 2016 · Modern x86 CPUs break down the incoming instruction stream into micro-operations (uops 1) and then schedule these uops out-of-order as their inputs become ready. ARM. Input/Output) in order to focus on more important topics of assembly language. Beginning x86 disassembly – Understanding "imul" and "div" with Visual Studio 2017 In this series of posts, I’m going through the Open Security Training for beginning Assembly Language and thus am putting my own spin on things to enhance my knowledge of x86 disassembly. Mar 4, 2015 · Hence, the C compiler only needs what imul provides, and since imul is easier to use than mul, the C compiler uses imul to avoid needing mov instructions to get data into / out of eax. pf 0F po so o proc st m rl x mnemonic op1 op2 op3 op4 iext tested f modif f def f undef f Oct 13, 2019 · Note that as commented, MASM is ignores the []. An example Given c = b * (b + a) where a, b and c are global integers. Here is an example: mov eax, 0x0fffffff mov ebx, 0x0fffffff imul eax, ebx <---- eax?? It seems that as 0x0fffffff * 0x0fffffff is equal to 0xffffffe0000001, greater than 0xffffffff, so eax should be assigned to 0xe0000001 after the imul. e. Arithmetic and logic add src, dest Add src to dest. So we think them as 16-bit signed numbers (so -2^15 <= bb <= 2^15-1 ) and, the multiplication set the CF / OF bit on if at least one sum (in the multiplication algorithm) get the CF / OF bit on. See full list on cs. Example: IMUL 0xA3 2 0xA0 0xA1 MPC BS3 C3 P3 P1 P0 ----- IMUL 0 0 00 A1 A0 - 0 0 00 00 00 - 0 0 00 00 00 - 0 0 00 00 00 - 0 0 00 00 00 - 2 0 A3 00 00 According to Agner Fog's testing (and other stuff like AIDA64) Intel CPUs since Core2 have had imul r32,r32, imm latency of 3c, throughput one per 1c. In this video, you will learn how to apply signed and unsigned multiplication instructions through MUL and IMUL in x86. The first form is of the kind n×n→2 n, meaning that it produces a result twice the size of the operands - we know from the theory why this makes sense. A word of advice, particularly to those who learn only the instructions noted above: Jul 13, 2019 · Because non-widening multiplication in 2's complement is the same for both signed and unsigned types, compilers will almost always use multi-operand imul as it's faster and more flexible. Instead MASM goes by the type for a label. imul clears the overflow and carry flags under the following conditions: Table 2-5 Clearing OR and CF Flags -- imul Dec 30, 2020 · For example, the "zero flag", or ZF for short, indicates whether the last operation that could have affected the flag, resulted in a 0. GCC implemented this formula a bit differently though. Multiplication imul imul eax, [var] ; multiply the contents of EAX by the ; 32-bit contents of the memory location ; var. Since this is the case you don't have to worry about remainders. mul. Quotient saved to EAX, remainder in EDX idiv DWORD PTR [ var ] ; EDX:EAX / 32-bit value stored at memory location var. May 10, 2015 · The purpose of cmov is to allow software (in some cases) to avoid a branch. Multiplications are expensive operations. add rax, 10 ; adds 10 to the rax register But mnemonics like mul and div only have a single operand - source - with the destination being hardcoded as rax. Jul 19, 2019 · On all modern x86 CPUs, LEA is a single uop. On Unix/Linux systems, the kernel delivers IMULInstruction • The IMULinstruction multiplies an 8-, 16, or 32-bit signed operand by either the AL, AX or EAX register (depending on the operand’s size). While the basic idea is clear, I'd like to know the specific details of how ready instructions are scheduled, since it impacts micro-optimization decisions. One-operand form — This form is identical to that used by the MUL instruction. Examples are provided to demonstrate how each instruction affects registers like AX, DX, and flags like CF, OF. Store the result in EAX. As a second step, since C compilers use the multiple-operand form of imul a lot, Intel and AMD invest effort into making it as fast as possible. Oct 18, 2015 · The two/tree-operand versions of imul yield single-word results. c file as input and generates assembly code (X86, AT&amp;T syntax). ; lfence times 25 imul edx,edx ; lfence dec ecx jnz . When the one-operand form of imul is passed a 32 bit argument, it effectively means EAX * src where both EAX and the source operand are 32-bit registers or memory. Here's a contrived example to make the point. Feb 18, 2015 · idiv part of answer mov AX, FFCE idiv AH AX=FFCE. . The syntaxes are the following. There are no different versions of ADD and SUB for signed and unsigned numbers. js, which can index the x86 database into much more friendly representation that can be used to actually do something with it. IMUL r64, r/m64, imm8* r64 = r/m64 * sign-extended imm8 * If the first two operands are the same, the second one can be left out when using nasm or . nyu. not directly translated into machine language instructions instructions to the assembler to perform specified actions during the assembly process Examples include: Define constants Designate areas of memory for data storage MASM . It can be used for byte, word or dword operation. The flags on x86-64 live in a special register called rflags. The imul r, r/m and imul r, r/m, imm forms are single Multiplication imul imul eax, [var] ; multiply the contents of EAX by the ; 32-bit contents of the memory location ; var. x86-64 assembly language is a human-readable version of this machine code. See @phuclv's answer on problem in understanding mul & imul instructions of Assembly language for more details, and of course Intel's instruction set reference manuals for mul and imul: •Multiplying 64-bit numbers can produce a 128-bit result. imul esi, edi, 25 ; ESI ← EDI * 25 Division idiv not - bitvise logical not (flips all bits) neg - negation neg eax ; EAX ← - EAX Opcode Instruction Op/En 64-Bit Mode Compat/Leg Mode Description; F6 /4: MUL r/m8: M: Valid: Valid: Unsigned multiply (AX := AL ∗ r/m8). See their respective tag wikis, intel-syntax and att. Everyting is good, but when I try to execute a IDIVQ instruction, I get a Jan 13, 2012 · Your function looks a little bit complicated to me. Mar 28, 2009 · For example, on most recent x86 chips, the common forms of the imul instruction have a latency of 3 cycles, and internally only one execution unit can handle them (unlike add which usually has four add-capable units). Since Nehalem, 64-bit multiplies are also that fast. imul trivia: Only the upper half of the full-multiply (N x N => 2N) result differs between imul and mul. ous example programs. The only question is how much better it is than the alternatives. The x86 Architecture Lecture 24 Intel Manual, Vol. Oct 5, 2010 · imul dest, source1, immediate imul reg, r/m, imm ; showing what kind of operand is allowed The source1 operand (either a memory location or a register) is multiplied by the immediate operand (either an 8-bit or 16/32-bit constant) and the result is stored in the dest operand (a 16, 32 or 64-bit register). _mydiv: xor %rdx, %rdx ; clear high bits of dividend mov %rdi, %rax ; copy dividend argument into rax idiv %rsi ; divide by divisor argument ret ; return (quotient is in rax) Mar 13, 2018 · And given x86 addressing modes, you just need to scale z by the struct size (with one imul or shift), then [x + scaled_z + offset] is a valid addressing mode where x and scaled_z are registers. But multiplying Aug 22, 2020 · And also why INT_MIN / -1 is C undefined behaviour: it overflows the signed quotient on 2's complement systems like x86. Therefore, if Emu8086 actually emulates the Intel 8086, then it should emulate both of these instructions, too. Jul 31, 2021 · x86-64 Assembly Language Programming with Ubuntu (Jorgensen) For example, assuming the following data declarations: (mul) and signed multiplication (imul). 4. Use of IMUL. If you add 1 to AL=254, the Carry will be 0. x86 idiv does indeed fault in this case. Any argument in stack of higher address than entry ESP is pushed by caller before the call is made; in this example, the first argument is at offset +4 from ESP (EIP is 4 bytes wide), plus 4 more bytes once the EBP is pushed on the stack. May 7, 2022 · In x86 assembly, most instructions have the following syntax: operation dest, source For example, add looks like. x86-64 has hundreds of instructions, and compiling programs to the most efficient machine code requires a good understanding of all of them–indeed, the fastest C compiler Mar 5, 2017 · If you want to learn Assembly language for Intel x86 Architecture I fully recommend Assembly Language for x86 Processors by Kip R. Multiplying two 64-bit integers results in a 128-bit integer therefore, IMUl will take the two arguments and multiply them then place the low 64-bits of the result in %rax and the high bits in %rdx. edu https://cs. Signed multiplication of 2 operands. DATA, and . imulS, D D ← D * S •If you specify one operand, it multiplies that by %rax, and splits the product x86-64's only hardware division instructions are idiv and div. definition of negative is that their highest ranked bit equals 1, so FFCE is negative, because 1111 1111 1100 1110 and processor cannot work with negatives, we need to have positive, that means we negate this number (or processor does this based on highest ranked bit automaticallly) Dec 28, 2016 · The program count result subtraction or multiplication depending on the specified the last argument (example: ". imul clears the overflow and carry flags under the following conditions: Table 2-5 Clearing OR and CF Flags -- imul The MUL/IMUL Instruction. imul r, r/m and imul r, r/m, imm are both 3 cycle latency, one per 1c throughput on Intel SnB-family and AMD Ryzen, even for 64bit operand size. imul clears the overflow and carry flags under the following conditions: Table 2-5 Clearing OR and CF Flags -- imul May 21, 2012 · The answer to this question can be very easily found by looking at the proper page of the Intel 64 and IA-32 Instruction Set Reference. There is no dl=sign_bit(al) instruction because 8bit mul and div are special, and use ax instead of dl:al. For example. IMUL only works with signed numbers. Adding 64-bit integers on x86 requires only two instructions (add/adc thanks to the carry flag), same for 128-bit on x86-64. Start Small: Begin with simple programs and gradually increase complexity. How does x86-64 support this with only 64-bit registers? •If you specify two operands to imul, it multiplies them together and truncates until it fits in a 64-bit register. Jul 14, 2016 · Signed multiplication of 2 operands. Feb 9, 2014 · as the low half of the result is the same and C programs often produce results with the same size as the operands, modern x86 CPUs are often optimized for imul and it has many more forms (imul rs, rd1[, rd2]) while mul has only 1 form mul rx – Opcode Instruction 64-Bit Mode Compat/Leg Mode Description; D8 /1: FMUL m32fp: Valid: Valid: Multiply ST(0) by m32fp and store result in ST(0). And of course other CPUs from other vendors (or emulators) could do anything they want. This section should not be considered an exhaustive list of x86 instructions, but rather a useful subset. out 5 8 x" = 40). x86汇编mul、imul、div、idiv、cbw、cwd、cdq指令简介，代码先锋网，一个为软件开发程序员提供代码片段和技术文章聚合的网站。 X86汇编MUL、IMUL、DIV、IDIV、CBW、CWD、CDQ指令简介 - 代码先锋网 Hmm but in your example 4 in binary is 0100 but if we put that result in AL it would be 0000 0100 and would take the MSB (Which is 0). This would be better to write as mov $7, %edi. 0x009a as a signed 2's complement 16-bit integer represents +154. I then want to idiv that 64-bit result by n3. Aug 5, 2017 · For example: global _start section . 0xff9a as a signed 2's complement 16-bit integer represents 0xff9a - 0x10000 = -102. Jan 19, 2019 · It depends on the syntax of an assembler. The problem is that if the 64-bit signed result is large enough and/or if n3 is small enough, an overflow will result and idiv will throw a #DE exception. The operands can be positive or negative. REX + F6 /4: MUL r/m8 1: M Sep 22, 2019 · There is no imul instruction where the destination is memory -- so the last operand (the destination and first source) must be a register. 386 allows imul reg,reg non-widening multiply with no implicit registers. Nov 30, 2020 · Footnote 1: 186 allows imul dst, src, constant like imul ax, cx, 5, allowing any registers with no implicit use of AX or DX. For example, EAX used to be called accumulator since it was used to by a number of arithmetic operations, and ECX was known as Sep 20, 2024 · Example: MOV AX, 1234h - copies value 1234h into AX. This was the main reason for introducing the 2's complement coding. Both instructions affect the Carry and Overflow flag. Why did you expect the upper byte to be zero? That makes no sense for signed or unsigned. Performs a signed multiplication of two operands. May 24, 2017 · It's designed for additional processing (for example to write validators, assemblers, disassemblers), but it's also very easy to just open the database file and explore it. Note that in the above example, I copied ‘Z’ into AL and then moved AL to [EDX]. as a memory location (for example, a label). For example, if you have this code: cmp eax,ebx jne . (A previous version of this document recommended that all asm statements for x86 explicitly clobber cc. I want to imul n1 by n2 to get a 64-bit signed result. inc DWORD PTR [var] — add one to the 32-bit integer stored at location var imul — Integer Multiplication The imul instruction has two basic formats: two-operand (first two syntax listings above) and three-operand (last two syntax listings above). Koether Overview of the x86 Architecture Instruction Format Registers Data Types The Run-time Stack Assignment The General-Purpose Registers The registers eax, ebx, ecx, and edx may be accessed as 32-bit, 16-bit, or 8-bit registers. Jun 10, 2016 · As you can see by looking at a guide to the x86 instruction set (here or here), the mul and imul instructions have been supported since the 8086. Multiplication and division definitely care IF you are using a variable number of bits for example two 16 bit operands multiplying into a 32 bit result that has to know signed vs unsigned to sign extend or zero pad to do the operation. May 5, 2017 · You're using the three-operand variant of the imul instruction, which is defined in the instruction set reference as IMUL r64, r/m64, imm8 and means "Multiply the contents of r/m64 by imm8 and store the result in r64". imul cl does AX = AL * CL, producing a full 16-bit signed product from 8-bit signed inputs. • The instruction formats are: IMUL r/m8 IMUL r/m16 IMUL r/m32 EAX x r/m32 EDX EAX May 6, 2016 · Re: x86 assembly language - I have three 32-bit signed numbers: n1, n2, and n3. je target jne target Jump to target if the last comparison had the corresponding result (je: equality; jne: inequality). Use shifts for powers of 2, or Apr 26, 2015 · But for the first one, for example, we do (bb)^2 with imul. idiv src, dest Divide dest by src. intel_syntax noprefix mode which many GNU and Unix tools can use. 6B /r ib IMUL r16, r/m16, imm8 word register ← r/m16 ∗ sign-extended immediate byte. The three-operand form of imul executes a signed multiply of a 16- or 32-bit immediate by a register or memory word or long and stores the product in a specified register word or long. •Multiplying 64-bit numbers can produce a 128-bit result. Dec 7, 2018 · An imul rax,rdx instruction computes the product of a and b. Mostly we have two choices: intel-syntax and AT&T att syntaxes. intel_syntax noprefix . data, . Sep 18, 2014 · For example, Skylake sets ZF=0 regardless of output or previous value in a few test cases. (Dividing in Assembler x86 and Displaying Time in Assembly has an example of using aam in 16-bit mode). In this section, we will use the following notation: • <reg32>- means any 32-bit register described in Section 2, for example, ESI. The MUL and IMUL (Integer MULtiplication) instructions are used to multiply two signed (with IMUL) or unsigned (with MUL) numbers. Since the high part is not needed in this case, it is not mandatory to use IMUL. Dec 17, 2016 · Multiplication on x86/x64 never overflows when using the one operand form. Jan 16, 2016 · For example, if EAX contained 0x7FFFFFFF you'd get 0 in EDX, since the most significant bit of EAX is clear. 64-bit mode removed aam, which does 8-bit division by an immediate. data byteVariable DB -5 section . IA32 Assembly: lea instruction. Rotate instructions update Carry and Overflow flags and preserve the others, the Overflow flag is undefined after a rotate if the count is greater than one etc. idiv works as expected for me here with this function:. – An example of the bits in FLAGS set by the CPU is the Carry Flag. then when a modern CPU sees the jne branch it will take a guess about whether the branch will be taken or not taken, and then start speculatively executing instructions based on the guess. Dataare the same Question: Assembly language x86- MUL, IMUL, and DIV instructions can't be used. I am currently writing a simple C compiler, that takes a . When the operand is a byte, it is multiplied with AL register and when it is a word, it is multiplied with AX register. 0201 Machine Level – Assembly (x86-64) basics Oct 16, 2020 · Related: Is it possible to multiply by an immediate with mul in x86 Assembly? - the answer there now mentions that imul-immediate requires 186, but not really a duplicate. shr src, dest shl Sep 18, 2008 · Since you're on x86 you need 4 mull instructions. The x86 exception is #DE - divide exception. The syntax for the MUL/IMUL instructions is as follows −. On AMD Bulldozer-family, it's 4c or 6c latency, and one per 2c or one per 4c throughput. There is also another variant of this instruction, which takes a 32-bit immediate, but it is impossible to tell which variant Aug 11, 2014 · test(unsigned int, unsigned int): mov eax, edi imul eax, esi ret test(int, int): mov eax, edi imul eax, esi ret (actual GCC output with -O1) So signedness doesn't matter for multiplication (at least not for the kind of multiplication you use in C) and for some other operations, namely: Aug 27, 2021 · Background and motivation Intel x86/x64 provides MUL/IMUL instructions that compute the low and high bits of a multiplication in a single instruction. . Apr 29, 2014 · For example, the INC and DEC instructions preserve the contents of the Carry Flag in EFLAGS. imul esi, edi, 25 ; ESI ← EDI * 25 Division idiv not - bitvise logical not (flips all bits) neg - negation neg eax ; EAX ← - EAX Find step-by-step Computer science solutions and your answer to the following textbook question: The x86 includes the following instruction: IMUL op1, op2, immediate This instruction multiplies op2, which may be either register or memory, by the immediate operand value, and places the result in op1, which must be a register. Signature (x86-imul-op/en-rm proc-mode start-rip temp-rip prefixes rex-byte opcode modr/m sib x86) → x86 Aug 27, 2016 · The other ways to nest the loops might be better or worse for performance with large matrices, but the only really good ways to do this (for large matrices) involve transposing one of the input matrices so you can loop over contiguous memory elements in both matrices at once (since transposing costs O(n^2) time, but speeds up the O(n^3) step which traverses the transposed array repeatedly Aug 31, 2020 · A separate comment is that you have a lot of instructions like mov $7, %rdi where you operate on 64-bit registers. With the one-operand form, the product is stored exactly in the destination. The x86 multiplication instruction imul comes in two form: the full form and the partial form. Syntax. 3/4/16 1 x86 Assembly Crash Course Don Porter Registers ò Only variables available in assembly ò General Purpose Registers: ò EAX, EBX, ECX, EDX (32 bit) The other answer represented the output as Intel syntax rather than AT&T but the answer seems okay. It covers unsigned multiplication with MUL, signed multiplication with IMUL, unsigned division with DIV, and signed division with IDIV. You can't read that register directly, but you can, for example, save its contents to the stack using the pushfq instruction. l1: . It is understandable and implements some libraries (e. Are mul and imul exactly the same when multiplying a signed number to an unsigned number, or is there a difference between them? Jun 23, 2016 · I am confused with one semantics of 32-bit x86 imul opcode. – IMUL Instruction • IMUL (signed integer multiply ) multiplies an 8-, 16-, or 32-bit signed operand by either AL, AX, or EAX • Preserves the sign of the product by sign-extending it into the upper half of the destination register Example: multiply 48 * 4, using 8-bit operands: mov al,48 mov bl,4 imul bl ; AX = 00C0h, OF=1 Apr 7, 2016 · The extensions of x86 to 32bit and 64bit have made the mnemonics slightly ambiguous (but remember, other than cbw, the within-eax versions always end with an e for Extend). The result is the correct sign to suit the signs of the multiplicand and the multiplier. virginia. 1, Chapter 3 For example, hlt - 0 operands imul - 1, 2, or 3 operands Robb T. movsx eax, byte [bVar3] imul dword [dVar1] ; result in edx:eax shl rdx, 32 or rax, rdx ; result in rax Examples dec %eax — subtract one from the contents of EAX incl var(,1) — add one to the 32-bit integer stored at location var. On x86, multiplying two 32-bit integers to create a 64-bit integer can be done with the the MUL instruction resulting in a unsigned 64-bit in EDX:EAX, or 128-bit result in RDX:RAX pair. 6B /r ib IMUL r32, r/m32, imm8 doubleword register ← r/m32 ∗ sign-extended immediate byte. The CF and OF are set when the result cannot fit in the operands size. AH=FF. This example illustrates use of the x86-64 instructions and (Logical AND), or Oct 2, 2021 · IMUL reg64,reg64,imm8 X64 IMUL reg64,reg64,sbytedword X64,ND IMUL reg64,reg64,imm32 X64 IMUL reg64,reg64,imm X64,ND I don't know what the point of the ND entries are, but mov is the only instruction in x86-64 that can take a 64-bit immediate. Dec 26, 2015 · MUL and IMUL differ in the high part of the result (EDX). I could also Apr 16, 2020 · On entry of a function, ESP points to the return address pushed on the stack by the call instruction (that is, previous contents of EIP). (link to the book on Amazon). eax by ecx imul eax, ecx imull %ecx The document discusses various multiplication and division instructions in x86 assembly language. Jan 29, 2022 · Taking x86 for example: jg, jge, jl, jle = branch based on a signed comparison (They check for the SF flag) ja, jae, jb, jbe = branch based on a unsigned comparison (They check for the CF flag) Most instructions on a CPU will be the same for signed/unsigned operations, because we're using a Two's-Complement representation these days. imul ax ;16x16->32, Result is dx:ax imul rax ;64x64->128 Examples dec %eax — subtract one from the contents of EAX incl var(,1) — add one to the 32-bit integer stored at location var. not real x86 machine instructions. smp tagrs owjx svmp ueb njcifm wotoi sjg cchhf rmby