#268 "Vertical bar" in openMSX doesn't look like such on real MSX

[Manuel Bilderbeek]

The attached program fills the VRAM with a regular pattern while having the mode bits set to specific values. This looks quite different in openMSX compared to real MSX: in real MSX, you see a messy pattern, while on openMSX there are nice vertical bars. See also http://www.msx.org/forumtopicl7331.html

[Manuel Bilderbeek]

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Oh, this is what Giavotto says about the code:
> Here, there is a lot of code bloat... look at main and fillvram.... (not
> the assembly version, the C one)

[Manuel Bilderbeek]

Disk with program that demonstrates the bug

[Manuel Bilderbeek]

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File Added: modetest.dsk.gz

[Manuel Bilderbeek]

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A correction form the author:
> It's not true that "undocumented mode looks badly"..... should be:
> "Vertical bar looks different in sc5 mode with this test program compared with a real msx2."
> the problem arise BEFORE i set the illegal mode, not after

[Manuel Bilderbeek]

And this is how it looks on real MSX: http://img180.imagevenue.com/img.php?image=93326_modetest_122_342lo.jpg

On openMSX it looks very close to this, but the stripe-effect is not seen there.

[Manuel Bilderbeek]

Attached how it looks on real MSX and on openMSX (now):

So, openMSX is ALMOST there!

[Manuel Bilderbeek]

Lowering priority, because openMSX is really close to the real thing here (except for those vertical stripes).

[Manuel Bilderbeek]

To be clear: this is a case of accessing the VDP too fast (see http://www.msx.org/forum/development/msx-development/vdp-illegal-modes?page=3)
So, this improved a lot when VDP access timing got implemented.

[Manuel Bilderbeek]

This is the routine that makes the screen output (after setting up screen 5):

0354  ld     bc,#0000     01 00 00
0357  ld     l,c          69
0358  ld     h,b          60
0359  srl    h            CB 3C
035B  rr     l            CB 1D
035D  srl    h            CB 3C
035F  rr     l            CB 1D
0361  srl    h            CB 3C
0363  rr     l            CB 1D
0365  ld     a,l          7D
0366  and    #0f          E6 0F
0368  ld     (#305f),a    32 5F 30
036B  sla    a            CB 27
036D  sla    a            CB 27
036F  sla    a            CB 27
0371  sla    a            CB 27
0373  ld     e,a          5F
0374  ld     a,(#305f)    3A 5F 30
0377  or     e            B3
0378  ld     (#305f),a    32 5F 30
037B  out    (#98),a      D3 98
037D  out    (#98),a      D3 98
037F  out    (#98),a      D3 98
0381  out    (#98),a      D3 98
0383  out    (#98),a      D3 98
0385  out    (#98),a      D3 98
0387  out    (#98),a      D3 98
0389  out    (#98),a      D3 98
038B  ld     hl,#0008     21 08 00
038E  add    hl,bc        09
038F  push   hl           E5
0390  pop    bc           C1
0391  ld     de,#6a00     11 00 6A
0394  ld     l,c          69
0395  ld     h,b          60
0396  or     a            B7
0397  sbc    hl,de        ED 52
0399  jr     c,#0357      38 BC

In C it boils down to:

void fillvideoram()
{
        unsigned int j;
        static char b;
        SetVRMWR(0, 0);
        for (j=0;j<27136;j+=8)
        {
                b=(j/8) & 15;
                b|=b<<4;
                outp(0x98,b);
                outp(0x98,b);
                outp(0x98,b);
                outp(0x98,b);
                outp(0x98,b);
                outp(0x98,b);
                outp(0x98,b);
                outp(0x98,b);

        }

}

This is SetVRMWR:

01D9  push   de           D5
01DA  ld     a,c          79
01DB  rlc    d            CB 02
01DD  rla                 17
01DE  rlc    d            CB 02
01E0  rla                 17
01E1  srl    d            CB 3A
01E3  out    (#99),a      D3 99
01E5  ld     a,#8e        3E 8E
01E7  out    (#99),a      D3 99
01E9  ld     a,e          7B
01EA  srl    d            CB 3A
01EC  out    (#99),a      D3 99
01EE  ld     a,d          7A
01EF  or     #40          F6 40
01F1  out    (#99),a      D3 99
01F3  pop    de           D1
01F4  ret                 C9

Attached a version of the program that has the actual mode test NOPed out (the last 3 vdpsetreg calls in main are removed). This is for easier testing on MSX2 without pause button :)