https://en.wikipedia.org/wiki/Advanced_Power_Management
Older than ACPI and simpler.
By Microsoft in 1995. Spec seems to be in RTF format...
Can't find the URL. A Google cache: https://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=1&ved=0CB0QFjAAahUKEwj7qpLN_4XIAhWCVxoKHa_nAxY&url=http%3A%2F%2Fdownload.microsoft.com%2Fdownload%2F1%2F6%2F1%2F161ba512-40e2-4cc9-843a-923143f3456c%2FAPMV12.rtf&usg=AFQjCNHoCx8gHv-w08Dn_Aoy6Q3K3DLWRg&sig2=D_66xvI7Y2n1cvyB8d2Mmg