x86_64 assembler first steps function parameters

Okay, so far I’ve figured that there are six registers used for the first 6 parameters used in a function call. The below c program sets up the function which also uses RAX as the return value:


int asmparams(int, int, int, int, int, int);

int main(void) {
	int ret = asmparams(1, 2, 3, 4, 5, 6);
	printf("return:%d\n", ret);


This now expects my assembler function to take 6 arguments and a return value. Below is how those first 6 arguments are used in 64 bit Unix function calls to assembler.

		section	.text

global	asmparams:function

extern	printf

		push    rbp                             ; stack frame? x64

		push	rsi
		push	rdx
		push	rcx
		push	r8
		push	r9

        mov     rsi,rax
        mov     rdi,pf_msg 
        xor     rax,rax     
        call    printf  

        pop		r9
        pop		r8
        pop		rcx
        pop		rdx
        pop		rsi

        pop     rbp                             ; stack frame


		mov		rax, rdi
		call	printnum

		mov		rax, rsi
		call	printnum

		mov		rax, rdx
		call	printnum

		mov		rax, rcx
		call 	printnum

		mov		rax, r8
		call	printnum

		mov		rax, r9
		call	printnum

		mov		rax, -1		; return value

		section	.data

pf_msg	db		"%08x",10,0

When I’ve got some time to explore how the ‘Prologue’ and ‘Epilogue’, or beginning and ending of a function, I post about that. At least now parameters can be passed to assembler functions.

Things have changed a lot since the 680×0 days. Then you could use a single push/pop instruction to push/pop multiple registers.

Eclipse Oxygen and NASM

Tonight I set myself a mission of getting any IDE to build assembler files alongside my C/C++ code.

Find it here: Eclipse Oxygen and NASM

I tried NetBeans at first but the IDE isn’t good at all, actually quite useless for setting up NASM. I spent about half an hour trying this out and eventually gave up. Apparently there are plugins out there, but I wasn’t going to try them out as they were not part of the NetBeans official plugins.

So I look into Eclipse and initially I was put off because there was nothing in the official plugins. A few searches later and I got it. It was already built in to the CDT plugin for eclipse. I only had to make a minor alteration to get it working.

Now I can have a C/C++ project that will also automatically compile and link my assembler source files.

Here’s me thinking I was going to be stuck over the weekend finding this out. Took an hour. What’s next to move on to? Oh yeah, my project. he he…

SSE SIMD and ARM NEON research

I started off wanting to know the CPU cycles and possible cache misses from SSE SIMD instructions, but was kind of mind blown at what SIMD can actually do. There’s also a hell of a lot that the best compilers cannot do with C or C++ code with SIMD stuff. An example is getting the sign bits from each value in a SIMD register in EAX, which is damn handy for some math.

The SSE SIMD stuff is only the base line because there’s now SSE4 I believe. (just checked and it is. behind times I am) Being able to multiple math operations on a single register has tickled my interest for a long time. Tonight I decided to put some proper research into it. Until I got sidetracked with ARM NEON.

Anyway, besides the point and before I go onto the ARM stuff. Over the next few nights at least, I’m going to be testing out more assembler programming. This time using SIMD instructions and possibly being able to use maybe a noise algorithm. Later on in time, not over the next few nights, I will look at using this experience for 3d matrix calcs.

But… Then I looked deeper into the ARM NEON…

What I found with the NEON is that it is kind of like a hyper-threaded architecture. The cpu will run 2 NEON instructions per cycle but during CPU down things (stalls, waits, etc). I need to get deeper to undertsand that but it does sound very much like the way the hyper-threading works on the intel core processors. Still good.

Another thing I did like about the ARM assembler language is it just just so awesome. In standard assembler, you load registers, multiply one register by 2/4/8, add them and the last instruction grabs the result. In ARM, you load the basic registers,  and in one instruction you can offsets and bit manipulate to get the address and store the result. Crazy.

I’ll come back to ARM stuff later one. For now, I’ll be focusing on x86_64 stuff and all the SIMD stuff. Over the next few days I’ll run some test and hopefully post some test code. That is if I get something running.

Assembler PRNG (from Java to Asm)

So, I had a perfectly working PRNG in Java like this:

public class WLPRNG {
    long seed;
    public WLPRNG(long seed) { this.seed = seed; }
    public int nextInt() {
        long result = seed + 0x123defca;
        result = Long.rotateLeft(result, 19);
        result += 0xbead6789;
        result *= 0x1234567c;
        int temp = (int)result;
        result ^= 0x5ecdab73;
        result = Long.rotateLeft(result, 48);
        if (temp % 4 == 0) result *= 0x87650027;
        result += 13;
        seed = result;
        return (int)result;

    public byte nextByte() {
        return (byte)nextInt();

And I thought I’d test out the Assembler version like this:

; random number generator to be used in crypto transmission
; of sensitive data over the internet

; WLGfx 2017-Nov-19

        section .text

global  main

extern  printf

srand:  mov     [seed],rax                      ; set random seed

arand:  mov     rax,[seed]                      ; get seed
        mov     rbx,qword 0x023defca321acfed
        add     rax,rbx                         ; add 64 bit value
        rol     rax,19                          ; rotate bits
        mov     rbx,qword 0xbead6789
        add     rax,rbx                         ; another add
        imul    rax,qword 0x1234567c            ; a multiple this time
        mov     rbx,rax                         ; copy into rbx
        xor     rax,qword 0x5ecdab73            ; flip some bits
        rol     rax,48                          ; rotate bits again
        mov     rcx,rax                         ; copy to rcx
        and     rax,0x3                         ; mask and test with 0
        jnz     .notz                           ; 25% chance of other ops
        mov     rax,rbx
        add     rcx,rax
        mov     rbx,qword 0x87650027
        imul    rax,rbx
        jmp     .cont
.notz   mov     rax,rcx                         ; back into rax
.cont   mov     [seed],rax                      ; store into seed
        and     rax,0xff                        ; return byte value only

main:   mov     rax,9                           ; set seed
        call    srand

        mov     dword[lc],10                    ; set loop counter

.loop   call    arand                           ; get random byte

        push    rbp                             ; stack frame
        mov     rsi,rax                         ; random number
        mov     rdi,pf_msg                      ; format string
        xor     rax,rax                         ; 0
        call    printf                          ; call printf
        pop     rbp                             ; stack frame

        sub     dword[lc],1                     ; dec loop counter
        jnz     .loop

        section .data

seed    dq      0,0,0,0                         ; random seed value 64 bit
lc      dd      10                              ; loop counter

pf_msg  db      "Number: 0x%02x",10,0

Using the build script:

nasm -f elf64 random.asm
gcc -o random random.o

Gives a sample output of:

 ~/dev/asm/tests $ ./ 
 ~/dev/asm/tests $ ./random 
Number: 0xe0
Number: 0x5b
Number: 0xca
Number: 0x7c
Number: 0xfc
Number: 0x2d
Number: 0x79
Number: 0xa5
Number: 0x62
Number: 0x7f

All I need to do now is to be able to link directly to C and C++ code. I’m currently reading up on threading in assembler, but it looks like the standard pthreads are just the same really.

There’s lot’s of potential for using assembler.

X86_64 printf 64 bit assembler test

So, I wanted to start playing about with assembler again. Mainly so I could use it for data encryption over the internet. Here’s a simple sample of printing 64 bit numbers as hex.

section .text
global  main

extern  printf

; use printf to print 64 bit hex string

_test:  push    rbp

        mov     rsi,0x1234567890abcdef
        mov     rdi,pf_msg
        xor     rax,rax
        call    printf

        pop     rbp

main:   call    _test
        mov     edx,len
        mov     ecx,msg
        mov     eax,4
        int     128
        ;mov     eax,1
        ;int     128
        xor     rax,rax

section .data

msg     db      "Hello world!",10
len     equ     $ - msg

; some testing stuff

pf_msg  db      "Register = %016llx", 10, 0

I set up a simple script to build the executable.

nasm -f elf64 test.asm
gcc -o test test.o

And the output is just…

Register = 1234567890abcdef
Hello world!