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Assembly

Assembly (also ASM) is, for any given hardware computing platform (ISA, basically a CPU architecture), the lowest level programming language that expresses typically a linear, unstructured (i.e. without nesting blocks of code) sequence of CPU instructions -- it maps (mostly) 1:1 to machine code (the actual binary CPU instructions) and basically only differs from the actual machine code by utilizing a more human readable form (it gives human friendly nicknames, or mnemonics, to different combinations of 1s and 0s). Assembly is converted by assembler into the the machine code, something akin a computer equivalent of the "DNA", the lowest level instructions for the computer. Assembly is similar to bytecode, but bytecode is meant to be interpreted or used as an intermediate representation in compilers while assembly represents actual native code run by hardware. In ancient times when there were no higher level languages (like C or Fortran) assembly was used to write computer programs -- nowadays most programmers no longer write in assembly (majority of zoomer "coders" probably never even touch anything close to it) because it's hard (takes a long time) and not portable, however programs written in assembly are known to be extremely fast as the programmer has absolute control over every single instruction (of course that is not to say you can't fuck up and write a slow program in assembly).

Assembly is NOT a single language, it differs for every architecture, i.e. every model of CPU has potentially different architecture, understands a different machine code and hence has a different assembly (though there are some standardized families of assembly like x86 that work on wide range of CPUs); therefore assembly is not portable (i.e. the program won't generally work on a different type of CPU or under a different OS)! And even the same kind of assembly language may have several different syntax formats that also create basically slightly different languages which differ e.g. in comment style, order of writing arguments and even instruction abbreviations (e.g. x86 can be written in Intel or AT&T syntax). For the reason of non-portability (and also for the fact that "assembly is hard") you mostly shouldn't write your programs directly in assembly but rather in a bit higher level language such as C (which can be compiled to any CPU's assembly). However you should know at least the very basics of programming in assembly as a good programmer will come in contact with it sometimes, for example during hardcore optimization (many languages offer an option to embed inline assembly in specific places), debugging, reverse engineering, when writing a C compiler for a completely new platform or even when designing one's own new platform (you'll probably want to make your compiler generate native assembly, so you have to understand it). You should write at least one program in assembly -- it gives you a great insight into how a computer actually works and you'll get a better idea of how your high level programs translate to machine code (which may help you write better optimized code) and WHY your high level language looks the way it does.

OK, but why doesn't anyone make a portable assembly? Well, people do, they just usually call it a bytecode -- take a look at that. C is portable and low level, so it is often called a "portable assembly", though it still IS significantly higher in abstraction and won't usually give you the real assembly vibes. Forth may also be seen as close to such concept. ACTUALLY Dusk OS has something yet closer, called Harmonized Assembly Layer (see https://git.sr.ht/~vdupras/duskos/tree/master/fs/doc/hal.txt). Web assembly would also probably fit the definition.

The most common assembly languages you'll encounter nowadays are x86 (used by most desktop CPUs) and ARM (used by most mobile CPUs) -- both are used by proprietary hardware and though an assembly language itself cannot (as of yet) be copyrighted, the associated architectures may be "protected" (restricted) e.g. by patents (see also IP cores). RISC-V on the other hand is an "open" alternative, though not yet so wide spread. Other assembly languages include e.g. AVR (8bit CPUs used e.g. by some Arduinos) and PowerPC.

To be precise, a typical assembly language is actually more than a set of nicknames for machine code instructions, it may offer helpers such as macros (something akin the C preprocessor), pseudoinstructions (commands that look like instructions but actually translate to e.g. multiple instructions), comments, directives, automatic inference of opcode from operands, named labels for jumps (as writing literal jump addresses would be extremely tedious) etc. I.e. it is still much easier to write in assembly than to write pure machine code even if you knew all opcodes from memory. For the same reason remember that just replacing assembly mnemonics with binary machine code instructions is not yet enough to make an executable program! More things have to be done such as linking libraries and converting the result to some executable format such as elf which contains things like header with metainformation about the program etc.

How will programming in assembly differ from your mainstream high-level programming? Quite a lot, assembly is extremely low level, so you get no handholding or much programming "safety" (apart from e.g. CPU operation modes), you have to do everything yourself -- you'll be dealing with things such as function call conventions, interrupts, syscalls and their conventions, counting CPU cycles of individual instructions, looking up exact hexadecimal memory addresses, opcodes, defining memory segments, dealing with endianness, raw goto jumps, call frames etc. You have no branching (if-then-else), loops or functions, you make these yourself with gotos. You can't write expressions like (a + 3 * b) / 10, no, you have to write every step of how to evaluate this expression using registers, i.e. something like: load a to register A, load b to register B, multiply B by 3, add register B to A, divide A by 10. You don't have any data types, you have to know yourself that your variables really represent signed values so when you're dividing, you have to use signed divide instruction instead of unsigned divide -- if you mess this up, no one will tell you, your program simply won't work. And so on.

Typical Assembly Language

Assembly languages are usually unstructured, i.e. there are no control structures such as if or while statements: these have to be manually implemented using labels and jump (goto, branch) instructions. There may exist macros that mimic control structures. The typical look of an assembly program is however still a single column of instructions with arguments, one per line, each representing one machine instruction.

In assembly it is also common to blend program instructions and data, i.e. sometimes you create a label after which you just put bytes that will represent e.g. text strings or images and after that you start to write program instructions that work with these data, which will likely physically be placed this way (after the data) in the final program. This may cause quite nasty bugs if you by mistake jump to a place where data reside and try to treat them as instructions.

The working of the language reflects the actual hardware architecture -- most architectures are based on registers so usually there is a small number (something like 16) of registers which may be called something like R0 to R15, or A, B, C etc. Sometimes registers may even be subdivided (e.g. in x86 there is an eax 32bit register and half of it can be used as the ax 16bit register). These registers are the fastest available memory (faster than the main RAM memory, they are literally INSIDE the CPU, even in front of the cache) and are used to perform calculations. Some registers are general purpose and some are special: typically there will be e.g. the FLAGS register which holds various 1bit results of performed operations (e.g. overflow, zero result etc.). Some instructions may only work with some registers (e.g. there may be kind of a "pointer" register used to hold addresses along with instructions that work with this register, which is meant to implement arrays). Values can be moved between registers and the main memory (with instructions called something like move, load or store).

Writing instructions works similarly to how you call a function in high level language: you write its name and then its arguments, but in assembly things are more complicated because an instruction may for example only allow certain kinds of arguments -- it may e.g. allow a register and immediate constant (kind of a number literal/constant), but not e.g. two registers. You have to read the documentation for each instruction. While in high level language you may write general expressions as arguments (like myFunc(x + 2 * y,myFunc2())), here you can only pass specific values.

There are also no complex data types, assembly only works with numbers of different size, e.g. 16 bit integer, 32 bit integer etc. Strings are just sequences of numbers representing ASCII values, it is up to you whether you implement null terminated strings or Pascal style strings. Pointers are just numbers representing addresses. It is up to you whether you interpret a number as signed or unsigned (some instructions treat numbers as unsigned, some as signed, some don't care because it doesn't matter).

Instructions are typically written as three-letter abbreviations and follow some unwritten naming conventions so that different assembly languages at least look similar. Common instructions found in most assembly languages are for example:

Fun note: HCF -- halt and catch fire -- is a humorous nickname for instructions that just stop the CPU and wait for restart.

How To

For specific assembly language how tos see their own articles: x86, Arm etc.

On Unices the objdump utility from GNU binutils can be used to disassemble compiled programs, i.e view the instructions of the program in assembly (other tools like ndisasm can also be used). Use it e.g. as:

objdump -d my_compiled_program

Let's now write a simple Unix program in 64bit x86 assembly -- we'll be using AT&T syntax that's used by GNU. Write the following source code into a file named e.g. program.s:

.global   _start         # include the symbol in object file

str:
.ascii    "it works\n"   # the string data

.text 
_start:                  # execution starts here
  mov     $5,   %rbx     # store loop counter in rbx

.loop:
  # make a Linux "write" syscall:
                         # args to syscall will be passed in regs.
  mov     $1,   %rax     # says syscalls type (1 = write)
  mov     $1,   %rdi     # says file to write to (1 = stdout)
  mov     $str, %rsi     # says the address of the string to write
  mov     $9,   %rdx     # says how many bytes to write
  syscall                # makes the syscall

  sub     $1,   %rbx     # decrement loop counter
  cmp     $0,   %rbx     # compare it to 0
  jne     .loop          # if not equal, jump to start of the loop

  # make an "exit" syscall to properly terminate:
  mov     $60,  %rax     # says syscall type (60 = exit)
  mov     $0,   %rdi     # says return value (0 = success)
  syscall                # makes the syscall

The program just writes out it works five times: it uses a simple loop and a Unix system call for writing a string to standard output (i.e. it won't work on Windows and similar shit).

Now assembly source code can be manually assembled into executable by running assemblers like as or nasm to obtain the intermediate object file and then linking it with ld, but to assemble the above written code simply we may just use the gcc compiler which does everything for us:

gcc -nostdlib -no-pie -o program program.s

Now we can run the program with

./program

And we should see

it works
it works
it works
it works
it works

As an exercise you can objdump the final executable and see that the output basically matches the original source code. Furthermore try to disassemble some primitive C programs and see how a compiler e.g. makes if statements or functions into assembly.

Example

Let's take the following C code:

#include <stdio.h>

char incrementDigit(char d)
{
  return // remember this is basically an if statement
    d >= '0' && d < '9' ?
    d + 1 :
    '?';
}

int main(void)
{
  char c = getchar();
  putchar(incrementDigit(c));
  return 0;
}

We will now compile it to different assembly languages (you can do this e.g. with gcc -S my_program.c). This assembly will be pretty long as it will contain boilerplate and implementations of getchar and putchar from standard library, but we'll only be looking at the assembly corresponding to the above written code. Also note that the generated assembly will probably differ between compilers, their versions, flags such as optimization level etc. The code will be manually commented.

{ I used this online tool: https://godbolt.org. ~drummyfish }

{ Also not sure the comments are 100% correct, let me know if not. ~drummyfish }

The x86 assembly may look like this (to understand the weird juggling of values between registers see calling conventions):

incrementDigit:
  pushq   %rbp                   # save base pointer
  movq    %rsp, %rbp             # move base pointer to stack top
  movl    %edi, %eax             # move argument to eax
  movb    %al, -4(%rbp)          # and move it to local var.
  cmpb    $47, -4(%rbp)          # compare it to '0'
  jle     .L2                    # if <=, jump to .L2
  cmpb    $56, -4(%rbp)          # else compare to '9'
  jg      .L2                    # if >, jump to .L4
  movzbl  -4(%rbp), %eax         # else get the argument
  addl    $1, %eax               # add 1 to it
  jmp     .L4                    # jump to .L4
.L2:
  movl    $63, %eax              # move '?' to eax (return val.)
.L4:
  popq    %rbp                   # restore base pointer
  ret
  
main:
  pushq   %rbp                   # save base pointer
  movq    %rsp, %rbp             # move base pointer to stack top
  subq    $16, %rsp              # make space on stack
  call    getchar                # push ret. addr. and jump to func.
  movb    %al, -1(%rbp)          # store return val. to local var.
  movsbl  -1(%rbp), %eax         # move with sign extension
  movl    %eax, %edi             # arg. will be passed in edi
  call    incrementDigit
  movsbl  %al, %eax              # sign extend return val.
  movl    %eax, %edi             # pass arg. in edi again
  call    putchar
  movl    $0, %eax               # values are returned in eax
  leave
  ret

The ARM assembly may look like this:

incrementDigit:
  sub   sp, sp, #16              // make room on stack
  strb  w0, [sp, 15]             // load argument from w0 to local var.
  ldrb  w0, [sp, 15]             // load back to w0
  cmp   w0, 47                   // compare to '0'
  bls   .L2                      // branch to .L2 if <
  ldrb  w0, [sp, 15]             // load argument again to w0
  cmp   w0, 56                   // compare to '9'
  bhi   .L2                      // branch to .L2 if >=
  ldrb  w0, [sp, 15]             // load argument again to w0
  add   w0, w0, 1                // add 1 to it
  and   w0, w0, 255              // mask out lowest byte
  b     .L3                      // branch to .L3
.L2:
  mov   w0, 63                   // set w0 (ret. value) to '?'
.L3:
  add   sp, sp, 16               // shift stack pointer back
  ret
  
main:
  stp   x29, x30, [sp, -32]!     // shift stack and store x regs
  mov   x29, sp
  bl    getchar
  strb  w0, [sp, 31]             // store w0 (ret. val.) to local var. 
  ldrb  w0, [sp, 31]             // load it back to w0
  bl    incrementDigit
  and   w0, w0, 255              // mask out lowest byte
  bl    putchar
  mov   w0, 0                    // set ret. val. to 0
  ldp   x29, x30, [sp], 32       // restore x regs
  ret

The RISC-V assembly may look like this:

incrementDigit:
  addi    sp,sp,-32              # shift stack (make room)
  sw      s0,28(sp)              # save frame pointer
  addi    s0,sp,32               # shift frame pointer
  mv      a5,a0                  # get arg. from a0 to a5
  sb      a5,-17(s0)             # save to to local var.
  lbu     a4,-17(s0)             # get it to a4
  li      a5,47                  # load '0' to a4
  bleu    a4,a5,.L2              # branch to .L2 if a4 <= a5
  lbu     a4,-17(s0)             # load arg. again
  li      a5,56                  # load '9' to a5
  bgtu    a4,a5,.L2              # branch to .L2 if a4 > a5
  lbu     a5,-17(s0)             # load arg. again
  addi    a5,a5,1                # add 1 to it
  andi    a5,a5,0xff             # mask out the lowest byte
  j       .L3                    # jump to .L3
.L2:
  li      a5,63                  # load '?'
.L3:
  mv      a0,a5                  # move result to ret. val.
  lw      s0,28(sp)              # restore frame pointer
  addi    sp,sp,32               # pop stack
  jr      ra                     # jump to addr in ra
  
main:
  addi    sp,sp,-32              # shift stack (make room)
  sw      ra,28(sp)              # store ret. addr on stack
  sw      s0,24(sp)              # store stack frame pointer on stack
  addi    s0,sp,32               # shift frame pointer
  call    getchar
  mv      a5,a0                  # copy return val. to a5
  sb      a5,-17(s0)             # move a5 to local var
  lbu     a5,-17(s0)             # load it again to a5
  mv      a0,a5                  # move it to a0 (func. arg.)
  call    incrementDigit
  mv      a5,a0                  # copy return val. to a5
  mv      a0,a5                  # get it back to a0 (func. arg.)
  call    putchar
  li      a5,0                   # load 0 to a5
  mv      a0,a5                  # move it to a0 (ret. val.)
  lw      ra,28(sp)              # restore return addr.
  lw      s0,24(sp)              # restore frame pointer
  addi    sp,sp,32               # pop stack
  jr      ra                     # jump to addr in ra

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