File:  [NetBSD Developer Wiki] / wikisrc / examples / netbsd_assembly.mdwn
Revision 1.3: download - view: text, annotated - select for diffs
Sat Oct 13 08:35:49 2018 UTC (16 months, 1 week ago) by leot
Branches: MAIN
CVS tags: HEAD
Fix some spello.

Reported by graff on #NetBSD@Freenode.


[[!toc levels=3]]

#   Assembly? 

_Assembly_ is the programming language that gives direct access to the instructions and registers of the processor. A program called the _assembler_ compiles assembly language into machine code. NetBSD installs the GNU assembler "gas" into /usr/bin/as and this program assembles for the host processor architecture. 

A higher-level compiler like "gcc" acts as a preprocessor to the assembler, by translating code from C (or other language) to assembler. Just run cc -S yourfile.c and look at the output yourfile.s to see assembly code. A higher-level compiler can probably write better assembly code than a human programmer who knows assembly language. 

There remain a few reasons to use assembly language. For example: 

  * You need direct access to processor registers (for example, to set the stack pointer). 
  * You need direct access to processor instructions (like for vector arithmetic or for atomic operations). 
  * You want to improve or fix the higher-level compiler, assembler, or linker. 
  * You want to optimize code, because your higher-level compiler was not good enough. 
  * You want to learn assembly language. 

#   i386 

_i386_ architecture takes its name from the Intel 386, the first x86 processor to have a 32-bit mode. Other names for this architecture are: 

  * _IA-32_, which means Intel Architecture, 32 bit. 
  * _x86_, which can mean the 32-bit mode or the ancient 16-bit mode. 

The i386 assembly language is either AT&T syntax or Intel syntax. Most programmers seem to prefer the Intel syntax. 

##   nasm 

NASM (the Netwide Assembler) is an x86 assembler that uses the Intel syntax. It is easily available via [devel/nasm]( 

You can also use [devel/yasm]( with [devel/nasm]( syntax. 

###   Hello world, NetBSD/i386 
    ; Hello world, NetBSD/i386 4.0
    section .note.netbsd.ident progbits alloc noexec nowrite
    	dd 0x00000007 ; Name size
    	dd 0x00000004 ; Desc size
    	dd 0x00000001 ; value 0x01
    	db "NetBSD", 0x00, 0x00 ; "NetBSD\0\0"
    	db 400000003 ; __NetBSD_Version__ (please see <sys/param.h>)
    section .data
    	msg db "Hello world!", 0x0a ; "Hello world\n"
    	len equ $ - msg
    section .text
    	global _start
    	; write()
    	mov eax, 0x04 ; SYS_write
    	push len ; write(..., size_t nbytes)
    	push msg ; write(..., const void *buf, ...)
    	push 0x01 ; write(int fd, ...)
    	push 0x00
    	int 0x80
    	pop ebx
    	; exit()
    	mov eax, 0x01 ; SYS_exit
    	push 0x00 ; exit(int status)
    	push 0x00
    	int 0x80

###   How to compile and link 

To use the above code you need to compile and then link it: 
    $ nasm -f elf hello.asm
    $ ld -o hello hello.o
    $ ./hello
    Hello world!

##   gas 

_the portable GNU assembler_

It uses AT&T syntax and is designed after the 4.2BSD assembler. You can use it on many CPU architectures. 

    .section ".note.netbsd.ident", "a"
            .long   2f-1f
            .long   4f-3f
            .long   1
    1:      .asciz  "NetBSD"
    2:      .p2align 2
    3:      .long   400000000
    4:      .p2align 2
    .section .data
        data_items:                                # this is an array
            .long 3,39,41,21,42,34,42,23,38,37,15,37,16,17,18,25,23,12,31,2
            .set DATASIZE, ( . - data_items) / 4 - 1
    .section .text
        .globl _start
            movl    $0, %edi                        # zero the index register
            movl    $DATASIZE, %ecx                 # set ecx to number of items
            movl    data_items(,%ecx,4), %eax       # load first item
            movl    %eax, %ebx                      # its the biggest atm
            decl    %ecx                            # decrement counter
            movl    data_items(,%ecx,4), %eax       # step to next element
            cmpl    %eax, %ebx                      # is it greater?
            cmovll  %eax, %ebx                      # set ebx to greater if its less
     than cur. num.
            jecxz   end_prog                        # if we are at item 0 end iterat
            jmp     main_loop                       # again!
            pushl   %ebx                            # return largest number
            pushl   %ebx                            # BSD-ism (has to push twice?)
            movl    $1, %eax                        # call exit
            int     $0x80                           # kernel

#   powerpc 

_PowerPC_ processors appear inside multiple different hardware platforms; NetBSD has at least 11 ports, see [[Platforms#PowerPC]]. The easiest way to obtain a PowerPC machine is probably to acquire a used Macintosh, choosing from among the [supported models for NetBSD/macppc]( 

PowerPC processors have 32-bit registers and pointers and use big-endian byte order. 

  * A very few boards (not with NetBSD) run the PowerPC in little-endian mode to match the hardware. 
  * A few PowerPC processors also have a 64-bit mode. NetBSD 5.0 will support some Apple G5 machines with these processors, but only in 32-bit mode (see [ppcg5 project]( 

##   gas 

Here is an example of a program for gas: 
    ##  factorial.s
    ##  This program is in the public domain and has no copyright.
    ##  This is an example of an assembly program for NetBSD/powerpc.
    ##  It computes the factorial of NUMBER using unsigned 32-bit integers
    ##  and prints the answer to standard output.
    	.set	NUMBER, 10
    .section ".note.netbsd.ident", "a"
    	#  ELF note to identify me as a native NetBSD program
    	#  type = 0x01, desc = __NetBSD_Version__ from <sys/param.h>
    	.int	7			# length of name	
    	.int	4			# length of desc
    	.int	0x01			# type
    	.ascii	"NetBSD\0"		# name
    	.ascii	"\0"			# padding
    	.int	500000003		# desc
    .section ".data"
    	.fill	16			# buffer for decimal ASCII
    	.ascii	"\n"			# newline at end of ASCII
    .section ".text"
    	#  PowerPC instructions need an alignment of 4 bytes
    	.balign	4
    	.globl	_start
    	.type	_start, @function
    	#  compute factorial in %r31
    	li	%r0, NUMBER
    	mtctr	%r0			# ctr = number
    	li	%r31, 1			# %r31 = factorial
    	li	%r30, 1			# %r30 = next factor
    	mullw	%r31, %r31, %r30	# multiply %r31 by next factor
    	addi	%r30, %r30, 1		# increment next factor
    	bdnz+	factorial_loop		# loop ctr times
    	#  prepare to convert factorial %r31 to ASCII.
    	lis	%r9, decbufend@ha
    	la	%r4, decbufend@l(%r9)	# %r4 = decbufend
    	lis	%r8, decbuffer@ha
    	la	%r29, decbuffer@l(%r8)	# %r29 = decbuffer
    	li	%r5, 1			# %r5 = length of ASCII
    	#  Each loop iteration divides %r31 by 10 and writes digit to
    	#  position %r4. Formula (suggested by gcc) to divide by 10,
    	#                     0xcccccccd
    	#  is to multiply by ----------- = 0.100000000005821
    	#                    0x800000000
    	#  which is to multiply by 0xcccccccd, then shift right 35.
    	.set	numerator, 0xcccccccd
    	lis	%r9, numerator@ha
    	la	%r28, numerator@l(%r9)	# %r28 = numerator
    	cmpw	%r29, %r4		# start of buffer <=> position
    	beq-	buffer_overflow
    	#  begin %r9 = (%r31 / 10)
    	mulhwu	%r9, %r31, %r28		# %r9 = ((%r31 * %r28) >> 32)
    	addi	%r4, %r4, -1		# move %r4 to next position
    	srwi	%r9, %r9, 3		# %r9 = (%r9 >> 3) = %r31 / 10
    	mulli	%r8, %r9, 10		# %r8 = (%r31 / 10) * 10
    	sub	%r27, %r31, %r8		# %r27 = %r31 % 10 = digit
    	addi	%r27, %r27, '0		# convert digit to ASCII
    	addi	%r5, %r5, 1		# count this ASCII digit
    	stb	%r27, 0(%r4)		# write ASCII digit to buffer
    	mr.	%r31, %r9		# %r31 /= 10, %r31 <=> 0
    	bne+	decloop			# loop until %r31 == 0
    	#  write(2) our factorial to standard output
    	li	%r0, 4			# SYS_write from <sys/syscall.h>
    	li	%r3, 1			# standard output
    	##	%r4			# buffer
    	##	%r5			# size of buffer
    	#  exit(2)
    	li	%r0, 1			# SYS_exit from <sys/syscall.h>
    	li	%r3, 0			# exit status
    	.size	_start, . - _start

With a NetBSD/powerpc system, you can run this program using 
    $ as -o factorial.o factorial.s
    $ ld -o factorial factorial.o
    $ ./factorial

##   Useful Documents 

To learn about PowerPC assembly language, here are two documents to start with. 

  * IBM developerWorks. [PowerPC Assembly]( This is a very good introduction to PowerPC assembly. It provides and explains the Hello World example (but using a Linux system call). 
  * SunSoft and IBM. [System V Application Binary Interface, PowerPC Processor Supplement]( (PDF file, hosted by Linux Foundation). This is the specification for 32-bit PowerPC code in ELF systems. It establishes %r1 as the stack pointer and describes the stack layout. It explains the C calling conventions, how to pass arguments to and return values from C functions, how to align data structures, and which registers to save to the stack. 

    * NetBSD, Linux and OpenBSD (and FreeBSD?) all use ELF with PowerPC and all follow this specification, with a few deviations and extensions. 

##   Wiki Pages 

  * [[ELF Executables for PowerPC]]. This introduces assembly language with a commented example. 

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