Whatever the issue was that prevented you from combining microcode and initrd files successfully, you can now try doing that with upgraded mkinitrd package:
Code:
Thu Jun 29 20:55:09 UTC 2017
<...>
patches/packages/mkinitrd-1.4.10-x86_64-1_slack14.2.txz: Upgraded.
Added support for -P option and MICROCODE_ARCH in mkinitrd.conf to specify
a microcode archive to be prepended to the initrd for early CPU microcode
patching by the kernel. Thanks to SeB.