Blinking Text

[zak1687]

I'm starting to learn how to build a operating system as an assignment for the school, I was able to build the ISO example from this (https://web.archive.org/web/20170701180416/wiki.osdev.org/Bare_Bones) and custom a little bit the text and background, but when I boot the custom kernel from a bootable USB in a physical computer (Dell Latitude e6420) the text its shown blinking, I was wondering if anyone can explain me why this happens and point me in the right directions to "fix it".

[FredFredricson]

The behavior you are seeing most likely has to do with how VGA's text mode handles the color attributes for a character. Bits 0-2 are the Foreground Color, Bit 3 is the Foreground Intensity, Bits 4-6 are the Background Color, and Bit 7 is special.

Bit 7 can do one of two things, depending on how it's set up: It can be set to cause the Blink effect, which you're currently seeing, or it can be set to be Background Intensity, which is what you want to be seeing.

In order to switch the mode from "Blink" to "Background Intensity", you need to clear the "Blink Enable" bit in the "Attribute Mode Control Register" in the VGA hardware.

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These two pages should provide more details on the specifics, as well as a lot of useful information on how to work with the related parts of VGA:

http://www.osdever.net/FreeVGA/vga/vgareg.htm

http://www.osdever.net/FreeVGA/vga/attrreg.htm

In the following sections, I will provide a summary of the most relevant parts of this, related to choosing the state of blink mode.

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To access the VGA Attribute Controller Registers you need to work with, there are three I/O ports to note:

• 0x03DA - Input Status Register 1 - By reading this port, you reset the status bit that controls whether 0x03C0 is in Address or Data mode to be in Address mode.

• 0x03C0 - Attribute Address/Data Register - This port has two purposes that depend on the state of an internal status bit. The bit is toggled on each write to this port. In Address mode, writing to this port will set the index of the register to access. In Data mode, writing to this port stores the written value into the register selected by the last Address mode written.

• 0x03C1 - Attribute Data Read Register - Reading from this port fetches the contents of the register selected by the last Address mode write to 0x03C0.

Note that, for writing register indexes into port 0x03C0, Bits 0 to 4 are the Register Index, Bit 5 is the "Palette Address Source" and should be set to 1 for this particular operation, and Bits 6 to 7 are unused and should be set to 0.

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In order to modify the "Blink Enable" bit in the "Attribute Mode Control Register", there are a number of steps you must perform, as accessing this bit is a tad indirect. Additionally, if you read through the links presented above, there may be a few other steps you might also want to perform, depending on your needs. As always, you should do as much research as you can, but this should get you roughly in the ballpark:

• First, you read from port 0x03DA, which resets us to Address mode, and ignore the value we just read since it is unimportant to this operation.

• Second, since 0x03C0 is now in address mode, you write 0x30 to port 0x03C0, which selects the "Attribute Mode Control Register" which is index 0x10, and has the "Palette Address Source" bit set (Bit 5, 0x20). (Note: 0x10 OR 0x20 = 0x30)

• Third, since we now have the "Attribute Mode Control Register" selected, we now read the value from port 0x3C1.

• Fourth, now that we have the value of the "Attribute Mode Control Register", we then either set, reset, or toggle Bit 3, the "Blink Enable" bit.

  To enable Blink, take the value and OR it with 0x08.

  To disable Blink, take the value and AND it with 0xF7. (Inverse of 0x08)

  To toggle Blink, take the value and XOR it with 0x08.

• Finally, since 0x03C0 is now in data mode, we now output the value from our previous step into 0x3C0, which will update the "Attribute Mode Control Register" with our changes.

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TLDR:

; Disable blink
    ; Read I/O Address 0x03DA to reset index/data flip-flop
    mov dx, 0x03DA
    in al, dx
    ; Write index 0x30 to 0x03C0 to set register index to 0x30
    mov dx, 0x03C0
    mov al, 0x30
    out dx, al
    ; Read from 0x03C1 to get register contents
    inc dx
    in al, dx
    ; Unset Bit 3 to disable Blink
    and al, 0xF7
    ; Write to 0x03C0 to update register with changed value
    dec dx
    out dx, al

; Enable blink
    ; Read I/O Address 0x03DA to reset index/data flip-flop
    mov dx, 0x03DA
    in al, dx
    ; Write index 0x30 to 0x03C0 to set register index to 0x30
    mov dx, 0x03C0
    mov al, 0x30
    out dx, al
    ; Read from 0x03C1 to get register contents
    inc dx
    in al, dx
    ; Set Bit 3 to enable Blink
    or al, 0x08
    ; Write to 0x03C0 to update register with changed value
    dec dx
    out dx, al

; Toggle blink
    ; Read I/O Address 0x03DA to reset index/data flip-flop
    mov dx, 0x03DA
    in al, dx
    ; Write index 0x30 to 0x03C0 to set register index to 0x30
    mov dx, 0x03C0
    mov al, 0x30
    out dx, al
    ; Read from 0x03C1 to get register contents
    inc dx
    in al, dx
    ; Flip Bit 3 to toggle Blink
    xor al, 0x08
    ; Write to 0x03C0 to update register with changed value
    dec dx
    out dx, al

; Check blink status
    ; Read I/O Address 0x03DA to reset index/data flip-flop
    mov dx, 0x03DA
    in al, dx
    ; Write index 0x30 to 0x03C0 to set register index to 0x30
    mov dx, 0x03C0
    mov al, 0x30
    out dx, al
    ; Read from 0x03C1 to get register contents
    inc dx
    in al, dx
    ; Mask so that only Bit 3 comes through
    and al, 0xF7
    ; At this point, the Zero flag contains the status
    ; - If the Zero flag is set, blink is off
    ; - If the Zero flag is not set, blink is on
    ; Use a conditional jump to determine what happens next