Compatibility: VGA
Expects: AX    1017H
BX    first DAC register to read (0-00ffH)
CX    number of registers to read (0-00ffH)
ES:DX addr of a buffer to hold R,G,B values (CX*3 bytes long)
Returns: ES:DX buffer contains returned data
Info: This obtains the current 18-bit RGB values of a consecutive block
of DAC color registers.

On entry, the buffer at ES:DX must be large enough to hold the
requested data.  Each register takes 3 bytes, so the buffer must
be CX*3 bytes long.

On return, the table is laid out as Red,Green,Blue,
Red,Green,Blue, ... etc.  Only the low-order 6 bits of each byte
has meaning.  See INT 10H 1010H for background info on 18-bit RGB
values for DAC color registers.

Notes: Use INT 10H 1012H to program a block of DAC registers.
See INT 10H 1013H for a reason why you might tend to work with
16- or 64-register blocks.

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INT 10H 1017H: Read a Block of DAC Color Registers