Contents

  1. Assembly?
  2. i386
    1. nasm
      1. Hello world, NetBSD/i386
      2. How to compile and link
    2. gas
  3. powerpc
    1. gas
    2. Useful Documents
    3. Wiki Pages

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:

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:

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 a 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

_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 codes you need to compile and then link them:

$ nasm -f elf hello.asm
$ ld -o hello hello.o
$ ./hello
Hello world!

gas

the portable GNU assembler

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

Example:

.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

    _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

    main_loop:
        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
ion
        jmp     main_loop                       # again!

    end_prog:
        pushl   %ebx                            # return largest number
        pushl   %ebx                            # BSD-ism (has to push twice?)
        movl    $1, %eax                        # call exit
        int     $0x80                           # kernel
        ret

powerpc

PowerPC processors appear inside multiple different hardware platforms; NetBSD has at least 11 ports, see ?Platforms. 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.

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"

decbuffer:
    .fill   16          # buffer for decimal ASCII
decbufend:
    .ascii  "\n"            # newline at end of ASCII

.section ".text"

    #  PowerPC instructions need an alignment of 4 bytes
    .balign 4

    .globl  _start
    .type   _start, @function
_start:
    #  compute factorial in %r31
    li  %r0, NUMBER
    mtctr   %r0         # ctr = number
    li  %r31, 1         # %r31 = factorial
    li  %r30, 1         # %r30 = next factor
factorial_loop:
    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
decloop:
    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
    #  FALLTHROUGH

buffer_overflow:
    #  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
    sc

    #  exit(2)
    li  %r0, 1          # SYS_exit from <sys/syscall.h>
    li  %r3, 0          # exit status
    sc

    .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
3628800
$

Useful Documents

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

Wiki Pages

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