Recent Posts

21

Programming with NASM / Re: how to switch video modes (switching to VESA video modes too) in protected mode

« Last post by fredericopissarra on April 13, 2025, 01:01:59 PM »

I ran into the same headache back when I was experimenting with writing a small OS kernel for fun. Switching to VESA modes in protected mode easily becomes a rabbit hole. I had to do a bunch of hacks just to get the BIOS to cooperate. The legacy BIOS interrupt (INT 0x10) just won't fly once you're out of real mode, so I remember relying on setting up a trampoline to call into real mode temporarily or using something like a VBE 2.0+ Linear Framebuffer approach after querying video modes beforehand.

Yep... OR you could setup and use Virtual 8086 mode (NOT available in x86-64 mode!) to do it... Anyway, it is a nightmare.

Another problems are: Getting the Linear Framebuffer Physical address from ModeBlockInfo strucuture... Often this field is zeroed in VMs like QEMU, for example... And page switching, for double buffering... another nightmare.

This and the Protect Mode Interface, described in VBE 3.0 documentation usually are not present (not even in Option-ROM that a nVidia card puts there)...

22

Programming with NASM / Re: how to switch video modes (switching to VESA video modes too) in protected mode

« Last post by ace012k8 on April 09, 2025, 01:00:47 PM »

I ran into the same headache back when I was experimenting with writing a small OS kernel for fun. Switching to VESA modes in protected mode easily becomes a rabbit hole. I had to do a bunch of hacks just to get the BIOS to cooperate. The legacy BIOS interrupt (INT 0x10) just won't fly once you're out of real mode, so I remember relying on setting up a trampoline to call into real mode temporarily or using something like a VBE 2.0+ Linear Framebuffer approach after querying video modes beforehand.

For videos and demo recording, I always used a lightweight editor to polish up footage or build quick walkthroughs. If you’re working on dev-related content and want to make clean quick edits, especially on Windows, I picked up a solid one here: https://www.movavi.com/imovie-for-windows/. Easy to use and doesn’t overcomplicate things.

23

Programming with NASM / Re: Printing of floating point values

« Last post by andyz74 on March 27, 2025, 02:07:47 PM »

I read a bit about the comiss command and the flags that are set, if there were failures, but to be honest, I am absolutely not sure, if this is the right way.

24

Programming with NASM / Re: Printing of floating point values

« Last post by fredericopissarra on March 25, 2025, 09:44:06 PM »

Ah, I got it. You decrease the fractional part after each loop and check with the
Comiss xmm0, xmm3
If it gets no more smaller...   This is nice. :-)

Of... since it seems you didn't got the three potential bugs there here they are:

• What will happen if the value is negative?
• What will happen if the value is too big (like bigger then 2⁶⁴-1?
• Whay will happen if the value is a NaN?

There is a simple way to fix this... I'll leave it to you...

25

Programming with NASM / Re: Printing of floating point values

« Last post by andyz74 on March 23, 2025, 04:42:04 PM »

Ah, I got it. You decrease the fractional part after each loop and check with the
Comiss xmm0, xmm3
If it gets no more smaller...   This is nice. :-)

26

Programming with NASM / Re: Printing of floating point values

« Last post by andyz74 on March 22, 2025, 06:42:50 PM »

OK, I will look and test it these days and be like every time thankful for reading programs from other people, that I can learn. :-)

But for now, one question :
You have in your program the possibility to control the number of digits after the point. (with EDI) .
EDI = 0 makes ALL numbers after the point.

So, how do we now, how much ALL is? Is there a special sign, that we see, the end of the numbers?

Yes, I overlooked your code, but couldn't find the answer ... lack of knowledge. :-(

Greetz from Germany, Andy

27

Programming with NASM / Re: Printing of floating point values

« Last post by fredericopissarra on March 22, 2025, 12:38:03 PM »

Very good... Notice that values like 123.456789 aren't exact in floating point... You can tweak a little bit and create a routine to present the value in a desired precision... Here's an example (it has a small bug, but I'll leave it to you to discover and fix it):

Code: [Select]

; main.asm
  bits  64
  default rel

  section .text

  extern printfloat

  global  _start
_start:
  movss xmm0, [value]
  xor   edi, edi      ; maximum precision.
  call  printfloat

  ; Prints a newline.
  mov   eax,1
  mov   edi,eax
  lea   rsi,[crlf]
  mov   edx,eax
  syscall

  ; Exits the program with errorcode 0.
  mov   eax,60
  xor   edi,edi
  syscall

  section .rodata

  align 4
value:
  dd  123.45678         ; this isn't exact in floating point!

crlf:
  db  `\n`

  section .note.GNU-stack noexec


Code: [Select]

; float.asm
  bits  64
  default rel

  section .text

  global  printfloat

; Input: XMM0 = x; EDI = precision (maximum # of fractional digits - 0 if all).
printfloat:
  cvttss2si r9, xmm0
  mov       byte [rsp-1], 0
  mov       r8d, edi
  lea       rsi, [rsp-1]
  mov       r10d, 0xcccccccd      ; 1/10 scaled.
  mov       edx, r9d

  align 4
.loop1:
  mov       eax, edx
  mov       ecx, edx
  sub       rsi, 1

  imul      rax, r10
  shr       rax, 35               ; EAX = n / 10.
  lea       edi, [rax+rax*4]
  add       edi, edi
  sub       ecx, edi              ; ECX = n % 10.

  add       ecx, '0'
  mov       [rsi], cl
  mov       ecx, edx
  mov       edx, eax
  cmp       ecx, 9                ; reminder sill > 9?
  ja        .loop1                ; yes, stay in the loop.

  mov       eax, 1
  mov       rdx, rsp    ; calc the string size.
  sub       rdx, rsi    ;
  mov       edi, eax
  syscall

  mov       eax,1
  mov       edi,eax
  mov       edx,eax
  lea       rsi, [dot]
  syscall

  pxor      xmm1, xmm1
  mov       r9d, r9d
  cvtsi2ss  xmm1, r9
  subss     xmm0, xmm1
  pxor      xmm1, xmm1
  cvttss2si rdx, xmm0
  mov       edx, edx
  cvtsi2ss  xmm1, rdx
  subss     xmm0, xmm1

  movss     xmm2, [ten]
  mov       r9d, r8d
  lea       rsi, [rsp-1]
  pxor      xmm3, xmm3

  align 4
.loop2:
  mulss     xmm0, xmm2
  pxor      xmm1, xmm1
  mov       edx, edi
  cvttss2si ecx, xmm0
  movsx     eax, cl
  add       ecx, '0'
  cvtsi2ss  xmm1, eax
  mov       [rsi], cl
  mov       eax, edi
  subss     xmm0, xmm1
  syscall

  ; Precision testing...
  test      r8d, r8d
  je        .skip
  sub       r9d, 1
  je        .exit
.skip:

  ; if fractional part isn't zero yet...
  comiss    xmm0, xmm3
  jne       .loop2      ; stay in the loop.

.exit:
  ret

  section .rodata

dot:
  db  '.'

  align 4
ten:
  dd    10.0

  section .note.GNU-stack noexec


Code: [Select]

$ nasm -felf64 main.asm -o main.o
$ nasm -felf64 float.asm -o float.o
$ ld -s -o main main.o float.o
$ ./main
123.45677947998046875

[]s
Fred

28

Programming with NASM / Re: Printing of floating point values

« Last post by andyz74 on March 20, 2025, 05:12:15 PM »

As I am on my way to become the GodMaster of Assembly ... (just joking...;-) )   I gave it another try and it works now for me.  Just as example, that it can be done.

It is a quick and dirty solution, maybe for purpose of checking values while developing, in a macro or *.inc-file

(Linux, 64 bit)

Code: [Select]

; try to print floating-point values via sse2

; nasm -f elf64 float-print.asm && ld -o float-print float-print.o


[bits 64]

global _start
 
section .data
   
    val1 dq 123.454789  ;declare quad word (double precision)
zehn dq 10.0
ziffer db 0
crlf db 13,10
dezp db '.'

section .text

_start:
    call newline
    mov r9, 1 ; r9 starts to count the numbers after decimalpoint we want to write
    movsd xmm1, qword [val1] ; our value, we want to write
    movsd xmm2, xmm1
    cvttsd2si rax, xmm2 ; rax is now val1 without numbers after decimalpoint
    call print_dez
call dez_point
  repeat_it:
cvtsi2sd xmm3, rax ; load the truncated value in xmm3
    subsd xmm2, xmm3 ; xmm2 is now the value behind the decimalpoint
    mulsd xmm2, [zehn] ; multiplic. by ten. (so get one number before the decimalpoint
    cvttsd2si rax, xmm2 ; rax is the number before the decimalpoint (only one number)
    call print_dez ; print this number
    inc r9
    cmp r9, 5 ;  <===   check if the wanted amount of "Nachkommastellen" is reached
    jle repeat_it
   
call newline

mov rax,60
syscall ;end program





print_dez: ; in rax should be the number to write
push rax
push rbx
push rcx
push rdx

xor rcx,rcx
mov rbx,10

schl1:
xor rdx,rdx
div rbx
push rdx
inc rcx
cmp rax,0

jnz schl1

schl2:
pop rdx
add dl,30h
mov [ziffer],dl
push rcx
mov rax, 1 ; Funktionsnummer : schreiben
mov rdi, 1 ; auf STDOUT
mov rsi, ziffer ; was schreiben wir
mov rdx, 1 ; wieviel Zeichen
syscall
pop rcx
loop schl2

pop rdx
pop rcx
pop rbx
pop rax
ret


dez_point: ; just to write the point.
push rax
push rdx
mov rax, 1 ; Funktionsnummer : schreiben
mov rdi, 1 ; auf STDOUT
mov rsi, dezp ; was schreiben wir
mov rdx, 1 ; wieviele Zeichen
syscall
pop rdx
pop rax
ret


newline:
push rax
push rdx
mov rax, 1 ; Funktionsnummer : schreiben
mov rdi, 1 ; auf STDOUT
mov rsi, crlf ; was schreiben wir
mov rdx, 2 ; wieviele Zeichen
syscall
pop rdx
pop rax
ret


29

Other Discussion / Re: framebuffer problem

« Last post by fredericopissarra on March 15, 2025, 04:49:50 PM »

a) if i have declared my points as integer values like "mydata dq 100" or whatever, I should not load them like "movsd xmm0, qword [mydata]" but with  "cvtsi2sd xmm0, qword [mydata]"

If you intend to use this data as a double, yes.

b) if I really work with my data as integer, they will get after a few rotation-calculations very fast unexact by the truncation/rounding.  After a 360 degree-rotation in 360 steps I will have no more a cube, but some totally other formation. ;-)    => Will have to do this with floating-point (also the storing of the coordinates)

Floating point is ALWAYS an approximation. Most exact fractional values in decimal cannot be exactly represented in binary...

[]s
Fred

30

Other Discussion / Re: framebuffer problem

« Last post by andyz74 on March 10, 2025, 04:24:00 PM »

I found out the following by myself : 

a) if i have declared my points as integer values like "mydata dq 100" or whatever, I should not load them like "movsd xmm0, qword [mydata]" but with  "cvtsi2sd xmm0, qword [mydata]"

b) if I really work with my data as integer, they will get after a few rotation-calculations very fast unexact by the truncation/rounding.  After a 360 degree-rotation in 360 steps I will have no more a cube, but some totally other formation. ;-)    => Will have to do this with floating-point (also the storing of the coordinates)