Nasm Assembly Intro - Understanding Calling Conventions (64-bit)
http://cs.lmu.edu/~ray/notes/nasmtutorial/
NASM Tutorial
Scope of the Tutorial
This tutorial will show you how to write assembly language programs for Linux on the x86-64 architecture. (There is a little section on writing for OS X at the end, though.)
You will write both (1) standalone programs and (2) programs that integrate with C.
We won't get too fancy.
What about NASM Details?
NASM is an awesome assembler. The best one. But assembly language is complex. You need more than a tutorial. You need details. Lots of details.
Be ready to consult:
- The NASM Manual, which is pretty good!
- The Intel Processor Manuals
Your First Program
Make sure both nasm and gcc are installed, then save the following program as 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: ; write(1, message, 13) mov rax, 1 ; system call 1 is 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 ; exit(0) mov eax, 60 ; system call 60 is exit xor rdi, rdi ; exit code 0 syscall ; invoke operating system to exit message: db "Hello, World", 10 ; note the newline at the end
Assemble and run it.
$ nasm -felf64 hello.asm && ld hello.o && ./a.out Hello, World
Structure of a NASM Program
NASM is line-based. Most programs consist of directives followed by one or moresections. Lines can have an optionallabel. Most lines have aninstruction followed by zero or moreoperands.
Put your code in a section called .text.
Your First Few Instructions
There are hundreds of instructions. You can't learn them all at once. Just start with these:
mov x, y |
x ← y |
and x, y |
x ← x and y |
or x, y |
x ← x or y |
xor x, y |
x ← x xor y |
add x, y |
x ← x + y |
sub x, y |
x ← x – y |
inc x |
x ← x + 1 |
dec x |
x ← x – 1 |
syscall |
Invoke an operating system routine |
db |
A 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:
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 thebase; the register with the scale is called theindex.
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
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 callsmain, then it does some clean up, then it issues system call 60. So you just have to implementmain. We can do that in assembly:
; ---------------------------------------------------------------------------------------- ; Writes "Hola, mundo" to the console using a C library. Runs on Linux or any other system ; that does not use underscores for symbols in its C library. To assemble and run: ; ; 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
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 theAMD64 ABI Reference. You can also get this information fromWikipedia. 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.
- For integers and pointers,
- Additional parameters are pushed on the stack, right to left, and are to beremoved 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
rspmust 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,rspis not aligned. You have to make that extra space yourself, by pushing something or subtracting 8 fromrsp. - 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
raxorrdx:rax, and floating point values are returned inxmm0orxmm1: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:
; ----------------------------------------------------------------------------- ; 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 x |
Decrement rsp by the size of the operand, then store x in[rsp] |
pop x |
Move [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 label |
Push the address of the next instruction, then jump to the label |
ret |
Pop 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.
; ----------------------------------------------------------------------------- ; 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.
/* * 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 itsrflags. 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, y |
x ← y if the last operation did not overflow |
setc x |
x ← 1 if the last operation had a carry, but x ← 0 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, andset 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:
; ----------------------------------------------------------------------------- ; 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, callatoi. Here's a neat program to compute xy.
; ----------------------------------------------------------------------------- ; 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:
; ----------------------------------------------------------------------------- ; 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:
/* * 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.
; ----------------------------------------------------------------------------- ; 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 xmmreg, r/m32 |
xmmreg[63..0] ← intToDouble(r/m32) |
cvtsi2ss xmmreg, r/m32 |
xmmreg[31..0] ← intToFloat(r/m32) |
cvtsd2si reg32, xmmr/m64 |
reg32 ← doubleToInt(xmmr/m64) |
cvtss2si reg32, xmmr/m32 |
reg32 ← floatToInt(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.
; ---------------------------------------------------------------------------- ; 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:
/* * 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 or multiple operations at a time. The operations have the form:
op xmmreg_or_memory, xmmreg |
For floating point addition, the instructions are:
addpd |
do 2 double-precision additions |
addps |
do just one double-precision addition, using the low 64-bits of the register |
addsd |
do 4 single-precision additions |
addss |
do just one single-precision addition, using the low 32-bits of the register |
Here's a function that adds four floats at once:
; 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:
#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:
paddb |
do 16 byte-additions |
paddw |
do 8 word-additions |
paddd |
do 4 dword-additions |
paddq |
do 2 qword-additions |
paddsb |
do 16 byte-additions with signed saturation (80..7F) |
paddsw |
do 8 word-additions with signed saturation (8000..7F) |
paddusb |
do 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 usemovaps to move from memory. There are other ways, but we're not covering everything in this tutorial.
; ---------------------------------------------------------------------------------------- ; 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