A C++ Developer Learns Assembly

Now I've at least got it working on an old laptop with 1366x768 resolution :)

Well, not immediately. At first there was an entirely different problem: everything on the screen was garbled!

However it was easy to figure out why and fix it: depending on the hardware and resolution, lines in the framebuffer can have extra padding beyond what's needed for the visible pixels. That information is available in another info structure:

              ; Get size of line in framebuffer
              mov eax, 16                   ; sys_ioctl
              mov edi, [drw_fbfd]
              mov esi, 0x4602               ; FBIOGET_FSCREENINFO
              mov rdx, rsp
              syscall
              mov eax, [rsp+0x30]           ; bytes per line
              shr eax, 2                    ; convert to pixels
              mov [drw_fb_pitch], eax

There are only a few places in draw.asm that then need to be changed to multiply by this new variable instead of drw_fb_w.

3 comments

rjinman2년 전

Hi, thanks for the update. I’ll make this change when I get a chance. Thanks

rep_lodsb2년 전

I've now also figured out a solution to my first problem, though still not much of an idea of what really causes it. In drw_flush, after writing out the screen buffer, do this:

              push 0                        ; put X,Y offsets on stack
              sub rsp, 16                   ; 4 more dwords (don't care)
              mov eax, 16                   ; sys_ioctl
              mov edi, [drw_fbfd]
              mov esi, 0x4606               ; FBIOPAN_DISPLAY
              mov rdx, rsp                  ; ptr to structure on stack
              syscall
              add rsp, 24                   ; drop it
              ret

From what little documentation there is, this IOCTL takes a fb_var_screeninfo structure, but only pays attention to the x/y offset fields (so allocating 24 bytes for it should be enough). With the offsets set to zero, it now works perfectly on my machine.

rjinman2년 전

Thanks, I've made a new branch called display_fixes. Let me know if it works