You can do it in the beginning of your code by the following EXOS call, until EXOS interrupt is active:
               LD BC,100H+28 ;B=1 write
;C=28 number of system variable (BIAS)
                LD D,0c0h ;new value
                EXOS 16 ;handling EXOS variable
If EXOS interrupt killed then:
        ld      a,color/8
         out   (80h),a

I wrote color/8, becuase EXOS call and direct write to BIAS register is different, BIAS is grouped by 8 colours,
0 th group: 0-7
1 st group: 8-0fh
2 nd group:10h-17h
...
31th group: f8h-ffh

You can use the colour numbers for EXOS BIAS call, but the bias will be the same when you select 0 or when you select 5.
When you enter BIAS directly by register you select the group identifiers, if you select 2 then last 8 color of the palette will be the following:
10h,11h,12h,13h,14h,15h,16h,17h

well, result isn't what i expected for

It is not related to the palette issue, but if you do not use double buffering, then it can be beneficial to generate the video interrupt at the beginning of the bottom border instead of at VSYNC, this way you get the most time to update the screen without flickering or other artifacts. Since the interrupt is triggered only after the LPB with the VINT bit set, in this particular case it would need to be set on the first (200 lines long) LPB.

You can do it in the beginning of your code by the following EXOS call, until EXOS interrupt is active:

Code: [Select]

                LD BC,100H+28 ;B=1 write
;C=28 number of system variable (BIAS)
                LD D,0c0h ;new value
                EXOS 16 ;handling EXOS variable

Also, if interrupts are disabled soon after this EXOS call, it is best to execute two HALT instructions after the EXOS 16 to make sure the new BIAS is really set.

If not needed any fast BIAS changing trick in the program then I recommend using EXOS variable setting.

Using EXOS variables does have one disadvantage: the new setting will remain in effect even after quitting the program, unless it is set to zero again in the reset routine. Programs that exit to the EP logo with a non-black border color can be annoying, BIAS is mostly an issue with some badly written games (old Spectrum conversions) that assume it is zero.

Great work, thanks!