You can create a shortcut to assign affinity.
X:\Windows\System32\cmd.exe /C start /affinity Y game.exe
Y
is hexadecimal and is a bit mask:
0x1 - 0001 - Core0 0x2 - 0010 - Core1 0x3 - 0011 - Core1 & Core0 0x4 - 0100 - Core2 0x5 - 0101 - Core2 & Core0 0x6 - 0110 - Core2 & Core1 0x7 - 0111 - Core2 & Core1 & Core0 0x8 - 1000 - Core3 0x9 - 1001 - Core3 & Core0 0xA - 1010 - Core3 & Core1 0xB - 1011 - Core3 & Core1 & Core0 0xC - 1100 - Core3 & Core2 0xD - 1101 - Core3 & Core2 & Core0 0xE - 1110 - Core3 & Core2 & Core1 0xF - 1111 - Core3 & Core2 & Core1 & Core0