NASM Tutorial


nasm-logo.png

Yep, it’s a tutorial.

This tutorial will show you how to write assembly language programs on the x86-64 architecture.

You will write both (1) standalone programs and (2) programs that integrate with C.

We won’t get too fancy.

Your First Program

Before learning about nasm, let’s make sure you can type in and run programs.

Make sure both nasm and gcc are installed. Save one of the following programs as hello.asm, depending on your machine platform. Then run the program according to the given instructions.

If you are on a Linux-based OS:

hello.asm

; ----------------------------------------------------------------------------------------
; Writes "Hello, World" to the console using only system calls. Runs on 64-bit Linux only.
; To assemble and run:
;
; nasm -felf64 hello.asm && ld hello.o && ./a.out
; ---------------------------------------------------------------------------------------- global _start section .text
_start: mov rax, 1 ; system call for write mov rdi, 1 ; file handle 1 is stdout mov rsi, message ; address of string to output mov rdx, 13 ; number of bytes syscall ; invoke operating system to do the write mov rax, 60 ; system call for exit xor rdi, rdi ; exit code 0 syscall ; invoke operating system to exit section .data
message: db "Hello, World", 10 ; note the newline at the end
$ nasm -felf64 hello.asm && ld hello.o && ./a.out
Hello, World

If you are on macOS:

hello.asm

; ----------------------------------------------------------------------------------------
; Writes "Hello, World" to the console using only system calls. Runs on 64-bit macOS only.
; To assemble and run:
;
; nasm -fmacho64 hello.asm && ld hello.o && ./a.out
; ---------------------------------------------------------------------------------------- global start section .text
start: mov rax, 0x02000004 ; system call for write mov rdi, 1 ; file handle 1 is stdout mov rsi, message ; address of string to output mov rdx, 13 ; number of bytes syscall ; invoke operating system to do the write mov rax, 0x02000001 ; system call for exit xor rdi, rdi ; exit code 0 syscall ; invoke operating system to exit section .data
message: db "Hello, World", 10 ; note the newline at the end
$ nasm -fmacho64 hello.asm && ld hello.o && ./a.out
Hello, World

Exercise: Identify the differences between the two programs.

Structure of a NASM Program

NASM is line-based. Most programs consist of directives followed by one or more sections. Lines can have an optional label. Most lines have an instruction followed by zero or more operands.

nasmstructure.png

Generally, you put code in a section called .text and your constant data in a section called .data.

Details

NASM is an awesome assembler, but assembly language is complex. You need more than a tutorial. You need details. Lots of details. Be ready to consult:

Your First Few Instructions

There are hundreds of instructions. You can’t learn them all at once. Just start with these:

mov xy      xy
and x, yxx and y
or x, yxx or y
xor x, yxx xor y
add x, yxx + y
sub x, yxxy
inc xxx + 1
dec xxx – 1
syscallInvoke an operating system routine
dbA pseudo-instruction that declares bytes that will be in memory when the program runs

The Three Kinds of Operands

Register Operands

In this tutorial we only care about the integer registers and the xmm registers. You should already know what the registers are, but here is a quick review. The 16 integer registers are 64 bits wide and are called:

R0 R1 R2 R3 R4 R5 R6 R7 R8 R9 R10 R11 R12 R13 R14 R15
RAX RCX RDX RBX RSP RBP RSI RDI

(Note that 8 of the registers have alternate names.) You can treat the lowest 32-bits of each register as a register itself but using these names:

R0D R1D R2D R3D R4D R5D R6D R7D R8D R9D R10D R11D R12D R13D R14D R15D
EAX ECX EDX EBX ESP EBP ESI EDI

You can treat the lowest 16-bits of each register as a register itself but using these names:

R0W R1W R2W R3W R4W R5W R6W R7W R8W R9W R10W R11W R12W R13W R14W R15W
AX CX DX BX SP BP SI DI

You can treat the lowest 8-bits of each register as a register itself but using these names:

R0B R1B R2B R3B R4B R5B R6B R7B R8B R9B R10B R11B R12B R13B R14B R15B
AL CL DL BL SPL BPL SIL DIL

For historical reasons, bits 15 through 8 of R0..R3 are named:

AH CH DH BH

And finally, there are 16 XMM registers, each 128 bits wide, named:

XMM0 ... XMM15

Study this picture; hopefully it helps:

rdx.png

Memory Operands

These are the basic forms of addressing:

  • [ number ]
  • [ reg ]
  • [ reg + reg*scale ]      scale is 1, 2, 4, or 8 only
  • [ reg + number ]
  • [ reg + reg*scale + number ]

The number is called the displacement; the plain register is called the base; the register with the scale is called the index.

Examples:

[750] ; displacement only
[rbp] ; base register only
[rcx + rsi*4] ; base + index * scale
[rbp + rdx] ; scale is 1
[rbx - 8] ; displacement is -8
[rax + rdi*8 + 500] ; all four components
[rbx + counter] ; uses the address of the variable 'counter' as the displacement

Immediate Operands

These can be written in many ways. Here are some examples from the official docs.

200 ; decimal
0200 ; still decimal - the leading 0 does not make it octal
0200d ; explicitly decimal - d suffix
0d200 ; also decimal - 0d prefex
0c8h ; hex - h suffix, but leading 0 is required because c8h looks like a var
0xc8 ; hex - the classic 0x prefix
0hc8 ; hex - for some reason NASM likes 0h
310q ; octal - q suffix
0q310 ; octal - 0q prefix
11001000b ; binary - b suffix
0b1100_1000 ; binary - 0b prefix, and by the way, underscores are allowed

Instructions with two memory operands are extremely rare

In fact, we’ll not see any such instruction in this tutorial. Most of the basic instructions have only the following forms:

add reg, reg
add reg, mem
add reg, imm
add mem, reg
add mem, imm

Defining Data and Reserving Space

These examples come from Chapter 3 of the docs. To place data in memory:

 db 0x55 ; just the byte 0x55 db 0x55,0x56,0x57 ; three bytes in succession db 'a',0x55 ; character constants are OK db 'hello',13,10,'$' ; so are string constants dw 0x1234 ; 0x34 0x12 dw 'a' ; 0x61 0x00 (it's just a number) dw 'ab' ; 0x61 0x62 (character constant) dw 'abc' ; 0x61 0x62 0x63 0x00 (string) dd 0x12345678 ; 0x78 0x56 0x34 0x12 dd 1.234567e20 ; floating-point constant dq 0x123456789abcdef0 ; eight byte constant dq 1.234567e20 ; double-precision float dt 1.234567e20 ; extended-precision float

There are other forms; check the NASM docs. Later.

To reserve space (without initializing), you can use the following pseudo instructions. They should go in a section called .bss (you'll get an error if you try to use them in a .text section):

buffer: resb 64 ; reserve 64 bytes
wordvar: resw 1 ; reserve a word
realarray: resq 10 ; array of ten reals

Another Example

Here’s a macOS program to study:

triangle.asm

; ----------------------------------------------------------------------------------------
; This is an OSX console program that writes a little triangle of asterisks to standard
; output. Runs on macOS only.
;
; nasm -fmacho64 triangle.asm && gcc hola.o && ./a.out
; ---------------------------------------------------------------------------------------- global start section .text
start: mov rdx, output ; rdx holds address of next byte to write mov r8, 1 ; initial line length mov r9, 0 ; number of stars written on line so far
line: mov byte [rdx], '*' ; write single star inc rdx ; advance pointer to next cell to write inc r9 ; "count" number so far on line cmp r9, r8 ; did we reach the number of stars for this line? jne line ; not yet, keep writing on this line
lineDone: mov byte [rdx], 10 ; write a new line char inc rdx ; and move pointer to where next char goes inc r8 ; next line will be one char longer mov r9, 0 ; reset count of stars written on this line cmp r8, maxlines ; wait, did we already finish the last line? jng line ; if not, begin writing this line
done: mov rax, 0x02000004 ; system call for write mov rdi, 1 ; file handle 1 is stdout mov rsi, output ; address of string to output mov rdx, dataSize ; number of bytes syscall ; invoke operating system to do the write mov rax, 0x02000001 ; system call for exit xor rdi, rdi ; exit code 0 syscall ; invoke operating system to exit section .bss
maxlines equ 8
dataSize equ 44
output: resb dataSize
$ nasm -fmacho64 triangle.asm && ld triangle.o && ./a.out
*
**
***
****
*****
******
*******
********

New things in this example:

  • cmp does a comparison
  • je jumps to a label if the previous comparison was equal. We also have jne (jump if not equal), jl (jump if less), jnl (jump if not less), jg (jump if greater), jng (jump if not greater), jle (jump if less or equal), jnle (jump if not less or equal), jge (jump if greater or equal), jnge (jump if not greater or equal), and many more.
  • equ is actually not a real instruction. It simply defines an abbreviation for the assembler itself to use. (This is a profound idea.)
  • The .bss section is for writable data.

Using a C Library

Writing standalone programs with just system calls is cool, but rare. We would like to use the good stuff in the C library.

Remember how in C execution “starts” at the function main? That’s because the C library actually has the _start label inside itself! The code at _start does some initialization, then it calls main, then it does some clean up, then it issues the system call for exit. So you just have to implement main. We can do that in assembly!

If you have Linux, try this:

hola.asm

; ----------------------------------------------------------------------------------------
; Writes "Hola, mundo" to the console using a C library. Runs on Linux.
;
; nasm -felf64 hola.asm && gcc hola.o && ./a.out
; ---------------------------------------------------------------------------------------- global main extern puts section .text
main: ; This is called by the C library startup code mov rdi, message ; First integer (or pointer) argument in rdi call puts ; puts(message) ret ; Return from main back into C library wrapper
message: db "Hola, mundo", 0 ; Note strings must be terminated with 0 in C
$ nasm -felf64 hola.asm && gcc hola.o && ./a.out
Hola, mundo

Under macOS, it will look a little different:

hola.asm

; ---------------------------------------------------------------------------------------- ; This is an macOS console program that writes "Hola, mundo" on one line and then exits. ; It uses puts from the C library. To assemble and run: ; ; nasm -fmacho64 hola.asm && gcc hola.o && ./a.out ; ---------------------------------------------------------------------------------------- global _main extern _puts section .text _main: push rbx ; Call stack must be aligned lea rdi, [rel message] ; First argument is address of message call _puts ; puts(message) pop rbx ; Fix up stack before returning ret section .data message: db "Hola, mundo", 0 ; C strings need a zero byte at the end 
$ nasm -fmacho64 hola.asm && gcc hola.o && ./a.out
Hola, mundo

In macOS land, C functions (or any function that is exported from one module to another, really) must be prefixed with underscores. The call stack must be aligned on a 16-byte boundary (more on this later). And when accessing named variables, a rel prefix is required.

Understanding Calling Conventions

How did we know the argument to puts was supposed to go in RDI? Answer: there are a number of conventions that are followed regarding calls.

When writing code for 64-bit Linux that integrates with a C library, you must follow the calling conventions explained in the AMD64 ABI Reference. You can also get this information from Wikipedia. The most important points are:

  • From left to right, pass as many parameters as will fit in registers. The order in which registers are allocated, are:
    • For integers and pointers, rdi, rsi, rdx, rcx, r8, r9.
    • For floating-point (float, double), xmm0, xmm1, xmm2, xmm3, xmm4, xmm5, xmm6, xmm7.
  • Additional parameters are pushed on the stack, right to left, and are to be removed by the caller after the call.
  • After the parameters are pushed, the call instruction is made, so when the called function gets control, the return address is at [rsp], the first memory parameter is at [rsp+8], etc.
  • The stack pointer rsp must be aligned to a 16-byte boundary before making a call. Fine, but the process of making a call pushes the return address (8 bytes) on the stack, so when a function gets control, rsp is not aligned. You have to make that extra space yourself, by pushing something or subtracting 8 from rsp.
  • The only registers that the called function is required to preserve (the calle-save registers) are: rbp, rbx, r12, r13, r14, r15. All others are free to be changed by the called function.
  • The callee is also supposed to save the control bits of the XMCSR and the x87 control word, but x87 instructions are rare in 64-bit code so you probably don’t have to worry about this.
  • Integers are returned in rax or rdx:rax, and floating point values are returned in xmm0 or xmm1:xmm0.

Got that? No? What’s need is more examples, and practice.

Here is a program that illustrates how registers have to be saved and restored:

fib.asm

; -----------------------------------------------------------------------------
; A 64-bit Linux application that writes the first 90 Fibonacci numbers. To
; assemble and run:
;
; nasm -felf64 fib.asm && gcc fib.o && ./a.out
; ----------------------------------------------------------------------------- global main extern printf section .text
main: push rbx ; we have to save this since we use it mov ecx, 90 ; ecx will countdown to 0 xor rax, rax ; rax will hold the current number xor rbx, rbx ; rbx will hold the next number inc rbx ; rbx is originally 1
print: ; We need to call printf, but we are using rax, rbx, and rcx. printf ; may destroy rax and rcx so we will save these before the call and ; restore them afterwards. push rax ; caller-save register push rcx ; caller-save register mov rdi, format ; set 1st parameter (format) mov rsi, rax ; set 2nd parameter (current_number) xor rax, rax ; because printf is varargs ; Stack is already aligned because we pushed three 8 byte registers call printf ; printf(format, current_number) pop rcx ; restore caller-save register pop rax ; restore caller-save register mov rdx, rax ; save the current number mov rax, rbx ; next number is now current add rbx, rdx ; get the new next number dec ecx ; count down jnz print ; if not done counting, do some more pop rbx ; restore rbx before returning ret
format: db "%20ld", 10, 0
$ nasm -felf64 fib.asm && gcc fib.o && ./a.out 0 1 1 2 . . . 679891637638612258 1100087778366101931 1779979416004714189

We just saw some new instructions:

push xDecrement rsp by the size of the operand, then store x in [rsp]
pop xMove [rsp] into x, then increment rsp by the size of the operand
jnz label      If the processor’s Z (zero) flag, is set, jump to the given label
call labelPush the address of the next instruction, then jump to the label
retPop into the instruction pointer

Mixing C and Assembly Language

This program is just a simple function that takes in three integer parameters and returns the maximum value.

maxofthree.asm

; ----------------------------------------------------------------------------- ; A 64-bit function that returns the maximum value of its three 64-bit integer ; arguments. The function has signature: ; ; int64_t maxofthree(int64_t x, int64_t y, int64_t z) ; ; Note that the parameters have already been passed in rdi, rsi, and rdx. We ; just have to return the value in rax. ; ----------------------------------------------------------------------------- global maxofthree section .text maxofthree: mov rax, rdi ; result (rax) initially holds x cmp rax, rsi ; is x less than y? cmovl rax, rsi ; if so, set result to y cmp rax, rdx ; is max(x,y) less than z? cmovl rax, rdx ; if so, set result to z ret ; the max will be in rax 

Here is a C program that calls the assembly language function.

callmaxofthree.c

/* * A small program that illustrates how to call the maxofthree function we wrote in * assembly language. */ #include <stdio.h>
#include <inttypes.h> int64_t maxofthree(int64_t, int64_t, int64_t); int main() { printf("%ld\n", maxofthree(1, -4, -7)); printf("%ld\n", maxofthree(2, -6, 1)); printf("%ld\n", maxofthree(2, 3, 1)); printf("%ld\n", maxofthree(-2, 4, 3)); printf("%ld\n", maxofthree(2, -6, 5)); printf("%ld\n", maxofthree(2, 4, 6)); return 0;
}
$ nasm -felf64 maxofthree.asm && gcc callmaxofthree.c maxofthree.o && ./a.out
1
2
3
4
5
6

Conditional Instructions

After an arithmetic or logic instruction, or the compare instruction, cmp, the processor sets or clears bits in its rflags. The most interesting flags are:

  • s (sign)
  • z (zero)
  • c (carry)
  • o (overflow)

So after doing, say, an addition instruction, we can perform a jump, move, or set, based on the new flag settings. For example:

jz label         Jump to label L if the result of the operation was zero
cmovno x, yxy if the last operation did not overflow
setc xx1 if the last operation had a carry, but x0 otherwise (x must be a byte-size register or memory location)

The conditional instructions have three base forms: j for conditional jump, cmov for conditional move, and set for conditional set. The suffix of the instruction has one of the 30 forms: s ns z nz c nc o no p np pe po e ne l nl le nle g ng ge nge a na ae nae b nb be nbe.

Command Line Arguments

You know that in C, main is just a plain old function, and it has a couple parameters of its own:

int main(int argc, char** argv)

So, you guessed it, argc will end up in rdi, and argv (a pointer) will end up in rsi. Here is a program that uses this fact to simply echo the commandline arguments to a program, one per line:

echo.asm

; ----------------------------------------------------------------------------- ; A 64-bit program that displays its command line arguments, one per line. ; ; On entry, rdi will contain argc and rsi will contain argv. ; ----------------------------------------------------------------------------- global main extern puts section .text main: push rdi ; save registers that puts uses push rsi sub rsp, 8 ; must align stack before call mov rdi, [rsi] ; the argument string to display call puts ; print it add rsp, 8 ; restore %rsp to pre-aligned value pop rsi ; restore registers puts used pop rdi add rsi, 8 ; point to next argument dec rdi ; count down jnz main ; if not done counting keep going ret 
$ nasm -felf64 echo.asm && gcc echo.o && ./a.out dog 22 -zzz "hi there"
./a.out
dog
22
-zzz
hi there

A Longer Example

Note that as far as the C Library is concerned, command line arguments are always strings. If you want to treat them as integers, call atoi. Here’s a neat program to compute xy.

power.asm

; -----------------------------------------------------------------------------
; A 64-bit command line application to compute x^y.
;
; Syntax: power x y
; x and y are (32-bit) integers
; ----------------------------------------------------------------------------- global main extern printf extern puts extern atoi section .text
main: push	r12 ; save callee-save registers push	r13 push	r14 ; By pushing 3 registers our stack is already aligned for calls cmp rdi, 3 ; must have exactly two arguments jne error1 mov r12, rsi ; argv ; We will use ecx to count down form the exponent to zero, esi to hold the
; value of the base, and eax to hold the running product. mov rdi, [r12+16] ; argv[2] call atoi ; y in eax cmp eax, 0 ; disallow negative exponents jl error2 mov r13d, eax ; y in r13d mov rdi, [r12+8] ; argv call	atoi ; x in eax mov r14d, eax ; x in r14d mov eax, 1 ; start with answer = 1
check: test r13d, r13d ; we're counting y downto 0 jz gotit ; done imul eax, r14d ; multiply in another x dec r13d jmp check
gotit: ; print report on success mov rdi, answer movsxd rsi, eax xor rax, rax call printf jmp done
error1: ; print error message mov edi, badArgumentCount call puts jmp done
error2: ; print error message mov edi, negativeExponent call puts
done: ; restore saved registers pop r14 pop r13 pop r12 ret answer: db "%d", 10, 0
badArgumentCount: db "Requires exactly two arguments", 10, 0
negativeExponent: db "The exponent may not be negative", 10, 0
$ nasm -felf64 power.asm && gcc -o power power.o
$ ./power 2 19
524288
$ ./power 3 -8
The exponent may not be negative
$ ./power 1 500
1
$ ./power 1
Requires exactly two arguments

Floating Point Instructions

Floating-point arguments go int the xmm registers. Here is a simple function for summing the values in a double array:

sum.asm

; ----------------------------------------------------------------------------- ; A 64-bit function that returns the sum of the elements in a floating-point ; array. The function has prototype: ; ; double sum(double[] array, uint64_t length) ; ----------------------------------------------------------------------------- global sum section .text sum: xorpd xmm0, xmm0 ; initialize the sum to 0 cmp rsi, 0 ; special case for length = 0 je done next: addsd xmm0, [rdi] ; add in the current array element add rdi, 8 ; move to next array element dec rsi ; count down jnz next ; if not done counting, continue done: ret ; return value already in xmm0 

Note the floating point instructions have an sd suffix; that’s the most common one, but we’ll see some other ones later. Here is a C program that calls it:

callsum.c

/* * Illustrates how to call the sum function we wrote in assembly language. */ #include <stdio.h> #include <inttypes.h> double sum(double[], uint64_t); int main() { double test[] = { 40.5, 26.7, 21.9, 1.5, -40.5, -23.4 }; printf("%20.7f\n", sum(test, 6)); printf("%20.7f\n", sum(test, 2)); printf("%20.7f\n", sum(test, 0)); printf("%20.7f\n", sum(test, 3)); return 0; } 
$ nasm -felf64 sum.asm && gcc sum.o callsum.c && ./a.out 26.7000000 67.2000000 0.0000000 89.1000000

Data Sections

The text section is read-only on most operating systems, so you might find the need for a data section. On most operating systems, the data section is only for initialized data, and you have a special .bss section for uninitialized data. Here is a program that averages the command line arguments, expected to be integers, and displays the result as a floating point number.

average.asm

; ----------------------------------------------------------------------------- ; 64-bit program that treats all its command line arguments as integers and ; displays their average as a floating point number. This program uses a data ; section to store intermediate results, not that it has to, but only to ; illustrate how data sections are used. ; ----------------------------------------------------------------------------- global main extern atoi extern printf default rel section .text main: dec rdi ; argc-1, since we don't count program name jz nothingToAverage mov [count], rdi ; save number of real arguments accumulate: push rdi ; save register across call to atoi push rsi mov rdi, [rsi+rdi*8] ; argv[rdi] call atoi ; now rax has the int value of arg pop rsi ; restore registers after atoi call pop rdi add [sum], rax ; accumulate sum as we go dec rdi ; count down jnz accumulate ; more arguments? average: cvtsi2sd xmm0, [sum] cvtsi2sd xmm1, [count] divsd xmm0, xmm1 ; xmm0 is sum/count mov rdi, format ; 1st arg to printf mov rax, 1 ; printf is varargs, there is 1 non-int argument sub rsp, 8 ; align stack pointer call printf ; printf(format, sum/count) add rsp, 8 ; restore stack pointer ret nothingToAverage: mov rdi, error xor rax, rax call printf ret section .data count: dq 0 sum: dq 0 format: db "%g", 10, 0 error: db "There are no command line arguments to average", 10, 0 
$ nasm -felf64 average.asm && gcc average.o && ./a.out 19 8 21 -33
3.75
$ nasm -felf64 average.asm && gcc average.o && ./a.out
There are no command line arguments to average

This program highlighted some processor instructions that convert between integers and floating point values. A few of the most common are:

cvtsi2sd xmmregr/m32      xmmreg[63..0]intToDouble(r/m32)
cvtsi2ss xmmregr/m32xmmreg[31..0]intToFloat(r/m32)
cvtsd2si reg32xmmr/m64reg32doubleToInt(xmmr/m64)
cvtss2si reg32xmmr/m32reg32floatToInt(xmmr/m32)

Recursion

Perhaps surprisingly, there’s nothing out of the ordinary required to implement recursive functions. You just have to be careful to save registers, as usual. Pushing and popping around the recursive call is a typical strategy.

factorial.asm

; ---------------------------------------------------------------------------- ; An implementation of the recursive function: ; ; uint64_t factorial(uint64_t n) { ; return (n <= 1) ? 1 : n * factorial(n-1); ; } ; ---------------------------------------------------------------------------- global factorial section .text factorial: cmp rdi, 1 ; n <= 1? jnbe L1 ; if not, go do a recursive call mov rax, 1 ; otherwise return 1 ret L1: push rdi ; save n on stack (also aligns %rsp!) dec rdi ; n-1 call factorial ; factorial(n-1), result goes in %rax pop rdi ; restore n imul rax, rdi ; n * factorial(n-1), stored in %rax ret 

An example caller:

callfactorial.c

/* * An application that illustrates calling the factorial function defined elsewhere. */ #include <stdio.h>
#include <inttypes.h> uint64_t factorial(uint64_t n); int main() { for (uint64_t i = 0; i < 20; i++) { printf("factorial(%2lu) = %lu\n", i, factorial(i)); } return 0;
}
$ nasm -felf64 factorial.asm && gcc -std=c99 factorial.o callfactorial.c && ./a.out
factorial( 0) = 1
factorial( 1) = 1
factorial( 2) = 2
factorial( 3) = 6
factorial( 4) = 24
factorial( 5) = 120
factorial( 6) = 720
factorial( 7) = 5040
factorial( 8) = 40320
factorial( 9) = 362880
factorial(10) = 3628800
factorial(11) = 39916800
factorial(12) = 479001600
factorial(13) = 6227020800
factorial(14) = 87178291200
factorial(15) = 1307674368000
factorial(16) = 20922789888000
factorial(17) = 355687428096000
factorial(18) = 6402373705728000
factorial(19) = 121645100408832000

SIMD Parallelism

The XMM registers can do arithmetic on floating point values one operation at a time (scalar) or multiple operations at a time (packed). The operations have the form:

op xmmreg_or_memory, xmmreg

For floating point addition, the instructions are:

addpd     do 2 double-precision additions in parallel (add packed double)
addsddo just one double-precision addition, using the low 64-bits of the register (add scalar double)
addpsdo 4 single-precision additions in parallel (add packed single)
addssdo just one single-precision addition, using the low 32-bits of the register (add scalar single)

Here’s a function that adds four floats at once:

add_four_floats.asm

; void add_four_floats(float x[4], float y[4])
; x[i] += y[i] for i in range(0..4) global add_four_floats section .text add_four_floats: movdqa xmm0, [rdi] ; all four values of x movdqa xmm1, [rsi] ; all four values of y addps xmm0, xmm1 ; do all four sums in one shot movdqa [rdi], xmm0 ret

and a caller:

test_add_four_floats.c

#include <stdio.h>
void add_four_floats(float[], float[]); int main() { float x[] = {-29.750, 244.333, 887.29, 48.1E22}; float y[] = {29.750, 199.333, -8.29, 22.1E23}; add_four_floats(x, y); printf("%f\n%f\n%f\n%f\n", x[0], x[1], x[2], x[3]); return 0;
}

Also see this nice little x86 floating-point slide deck from Ray Seyfarth.

Saturated Arithmetic

The XMM registers can also do arithmetic on integers. The instructions have the form:

op xmmreg_or_memory, xmmreg

For integer addition, the instructions are:

paddbdo 16 byte-additions
paddwdo 8 word-additions
paddddo 4 dword-additions
paddqdo 2 qword-additions
paddsbdo 16 byte-additions with signed saturation (80..7F)
paddswdo 8 word-additions with signed saturation (8000..7F)
paddusbdo 16 byte-additions with unsigned saturation (00..FF)
paddusw     do 8 word-additions with unsigned saturation (00..FFFF)

Here’s an example. It also illustrates how you load the XMM registers. You can’t load immediate values; you have to use movaps to move from memory. There are other ways, but we’re not covering everything in this tutorial.

satexample.asm

; ----------------------------------------------------------------------------------------
; Example of signed saturated arithmetic.
; ---------------------------------------------------------------------------------------- global main extern printf section .text
main: push rbp movaps xmm0, [arg1] movaps xmm1, [arg2] paddsw xmm0, xmm1 movaps [result], xmm0 lea rdi, [format] mov esi, dword [result] mov edx, dword [result+4] mov ecx, dword [result+8] mov r8d, dword [result+12] xor rax, rax call printf pop rbp ret section .data align 16
arg1: dw 0x3544,0x24FF,0x7654,0x9A77,0xF677,0x9000,0xFFFF,0x0000
arg2: dw 0x7000,0x1000,0xC000,0x1000,0xB000,0xA000,0x1000,0x0000
result: dd 0, 0, 0, 0
format: db '%x%x%x%x',10,0

Graphics

TODO

Local Variables and Stack Frames

First, please read Eli Bendersky’s article That overview is more complete than my brief notes.

When a function is called the caller will first put the parameters in the correct registers then issue the call instruction. Additional parameters beyond those covered by the registers will be pushed on the stack prior to the call. The call instruction puts the return address on the top of stack. So if you have the function

int64_t example(int64_t x, int64_t y) { int64_t a, b, c; b = 7; return x * b + y;
}

Then on entry to the function, x will be in edi, y will be in esi, and the return address will be on the top of the stack. Where can we put the local variables? An easy choice is on the stack itself, though if you have enough regsters, use those.

If you are running on a machine that respect the standard ABI, you can leave rsp where it is and access the "extra parameters" and the local variables directly from rsp for example:

 +----------+ rsp-24 | a | +----------+ rsp-16 | b | +----------+ rsp-8 | c | +----------+ rsp | retaddr | +----------+ rsp+8 | caller's | | stack | | frame | | ... | +----------+

So our function looks like this:

 global example section .text
example: mov qword [rsp-16], 7 mov rax, rdi imul rax, [rsp+8] add rax, rsi ret

If our function were to make another call, you would have to adjust rsp to get out of the way at that time.

On Windows you can’t use this scheme because if an interrupt were to occur, everything above the stack pointer gets plastered. This doesn’t happen on most other operating systems because there is a "red zone" of 128 bytes past the stack pointer which is safe from these things. In this case, you can make room on the stack immediately:

example: sub rsp, 24

so our stack looks like this:

 +----------+ rsp | a | +----------+ rsp+8 | b | +----------+ rsp+16 | c | +----------+ rsp+24 | retaddr | +----------+ rsp+32 | caller's | | stack | | frame | | ... | +----------+

Here’s the function now. Note that we have to remember to replace the stack pointer before returning!

 global example section .text
example: sub rsp, 24 mov qword [rsp+8], 7 mov rax, rdi imul rax, [rsp+8] add rax, rsi add rsp, 24 ret

Using NASM on macOS

Hopefully you’ve gone through the whole tutorial above using a Linux-based operating system (or perhaps more correctly, and ELF64 system). There are pretty much only five thing to know to get these examples working under a 64-bit macOS system:

  • This object file format is macho64, not elf64.
  • The system call numbers are totally different.
  • Symbols shared between modules will be prefixed by underscores.
  • It seems that the gcc linker in macOS doesn’t allow absolute addressing unless you tweak some settings. So add default rel when you are referencing labeled memory locations, and always use lea to get your addresses.
  • Also, it appears that sometimes under Linux, the 16-bit stack alignment requirement is not enforced, but it appears to be always enforced under macOS.

So here’s the average program from above, written for macOS.

average.asm

; -----------------------------------------------------------------------------
; 64-bit program that treats all its command line arguments as integers and
; displays their average as a floating point number. This program uses a data
; section to store intermediate results, not that it has to, but only to
; illustrate how data sections are used.
;
; Designed for OS X. To assemble and run:
;
; nasm -fmacho64 average.asm && gcc average.o && ./a.out
; ----------------------------------------------------------------------------- global _main extern _atoi extern _printf default rel section .text
_main: push rbx ; we don't ever use this, but it is necesary ; to align the stack so we can call stuff dec rdi ; argc-1, since we don't count program name jz nothingToAverage mov [count], rdi ; save number of real arguments
accumulate: push rdi ; save register across call to atoi push rsi mov rdi, [rsi+rdi*8] ; argv[rdi] call _atoi ; now rax has the int value of arg pop rsi ; restore registers after atoi call pop rdi add [sum], rax ; accumulate sum as we go dec rdi ; count down jnz accumulate ; more arguments?
average: cvtsi2sd xmm0, [sum] cvtsi2sd xmm1, [count] divsd xmm0, xmm1 ; xmm0 is sum/count lea rdi, [format] ; 1st arg to printf mov rax, 1 ; printf is varargs, there is 1 non-int argument call _printf ; printf(format, sum/count) jmp done nothingToAverage: lea rdi, [error] xor rax, rax call _printf done: pop rbx ; undoes the stupid push at the beginning ret section .data
count: dq 0
sum: dq 0
format: db "%g", 10, 0
error: db "There are no command line arguments to average", 10, 0
$ nasm -fmacho64 average.asm && gcc average.o && ./a.out
There are no command line arguments to average
$ nasm -fmacho64 average.asm && gcc average.o && ./a.out 54.3
54
$ nasm -fmacho64 average.asm && gcc average.o && ./a.out 54.3 -4 -3 -25 455.1111
95.4

Using NASM on Windows

I’m not sure what the system calls are on Windows, but I do know that if you want to assemble and link with the C library, you have to understand the x64 conventions. Read them. You will learn such things as:

  • The first four integer parameters are passed in RCX, RDX, R8, and R9. The rest are to be pushed on the stack.
  • The callee must preserve RBX, RBP, RDI, RSI, RSP, R12, R13, R14, and R15.
  • The first four floating point parameters are passed in, you guessed it, XMM0, XMM1, XMM2, and XMM3.
  • Return values go in RAX or XMM0.

IMPORTANT: There’s one thing that’s really hard to find in any documentation: the x64 calling convention requires you to allocate 32 bytes of shadow space before each call, and remove it after your call. This means your “hello world” program looks like this:

hello.asm

; ----------------------------------------------------------------------------------------
; This is a Win64 console program that writes "Hello" on one line and then exits. It
; uses puts from the C library. To assemble and run:
;
; nasm -fwin64 hello.asm && gcc hello.obj && a
; ---------------------------------------------------------------------------------------- global main extern puts section .text
main: sub rsp, 20h ; Reserve the shadow space mov rcx, message ; First argument is address of message call puts ; puts(message) add rsp, 20h ; Remove shadow space ret
message: db 'Hello', 0 ; C strings need a zero byte at the end