Encode data in a multi-byte noop

The 0x66 indicates that the JMP (0xEA) refers to six bytes. The default is refering to 64K (16 bits) in real mode or to 32 bits in protected mode (if I recall well). Having it increased, it also includes the segment descriptor, the index of the segment either in the GDT or the LDT, which means, that this code is making what is traditionally called a "long jump": a jump that cross beyond segments in the x86 architecture. The segment, in this case, points to the second entry on the GDT. If you look before in that program, you'll likely see how the GDT is defined in terms of the segment starting address and length (look in the Intel manual to study the GDT and LDT tables, 32 bit entry describing each segment).

Your asm code is relativity slow because use stack end write 8 times to memory. Check this one...

procedure DecodePixels(EncPixels: Byte; var DecPixels: TDecodedPixels);
asm
  xor   ecx, ecx
  add   al, al
  rcl   ecx, 8
  add   al, al
  rcl   ecx, 8
  add   al, al
  rcl   ecx, 8
  add   al, al
  rcl   ecx, 1
  mov   [DecPixels + 4], ecx
  xor   ecx, ecx
  add   al, al
  rcl   ecx, 8
  add   al, al
  rcl   ecx, 8
  add   al, al
  rcl   ecx, 8
  add   al, al
  rcl   ecx, 1
  mov   [DecPixels], ecx
end;

Maybe is even faster than code with lookup table!

Improved version:

procedure DecodePixelsI(EncPixels: Byte; var DecPixels: TDecodedPixels);
asm
  mov   ecx, 0    //Faster than: xor   ecx, ecx
  add   al, al
  rcl   ch, 1
  add   al, al
  rcl   cl, 1
  ror   ecx, 16
  add   al, al
  rcl   ch, 1
  add   al, al
  rcl   cl, 1
  mov   [DecPixels + 4], ecx
  mov   ecx, 0    //Faster than: xor   ecx, ecx
  add   al, al
  rcl   ch, 1
  add   al, al
  rcl   cl, 1
  ror   ecx, 16
  add   al, al
  rcl   ch, 1
  add   al, al
  rcl   cl, 1
  mov   [DecPixels], ecx
end;

Version 3:

procedure DecodePixelsX(EncPixels: Byte; var DecPixels: TDecodedPixels);
asm
  add   al, al
  setc  byte ptr[DecPixels + 7]
  add   al, al
  setc  byte ptr[DecPixels + 6]
  add   al, al
  setc  byte ptr[DecPixels + 5]
  add   al, al
  setc  byte ptr[DecPixels + 4]
  add   al, al
  setc  byte ptr[DecPixels + 3]
  add   al, al
  setc  byte ptr[DecPixels + 2]
  add   al, al
  setc  byte ptr[DecPixels + 1]
  setnz byte ptr[DecPixels]
end;

Version 4:

const Uint32DecPix : array [0..15] of cardinal = (
  $00000000, $00000001, $00000100, $00000101,
  $00010000, $00010001, $00010100, $00010101,
  $01000000, $01000001, $01000100, $01000101,
  $01010000, $01010001, $01010100, $01010101
  );

procedure DecodePixelsY(EncPixels: byte; var DecPixels: TDecodedPixels); inline;
begin
  pcardinal(@DecPixels)^ := Uint32DecPix[EncPixels and $0F];
  pcardinal(cardinal(@DecPixels) + 4)^ := Uint32DecPix[(EncPixels and $F0) shr 4];
end;