I mailed this to "linux-activists".  Others may be interested...
(There is a laptops thread on linux-activists)

quoted text
> Date: Fri, 28 Aug 92 23:27:58 -0400
> From: tytso@athena.mit.edu (Theodore Ts'o)
> Sender: owner-linux-activists@joker.cs.hut.fi
> X-Note1: Remember to put 'X-Mn-Key: normal' to your mail body or header
> X-Mn-Key: NORMAL
> Address: 1 Amherst St., Cambridge, MA 02139
> Phone: (617) 253-8091
>
>    Date:      Sat, 29 Aug 1992 06:01:15 +0300
>    From: Ed Carp <erc@khijol.upl.com>
>
>    I wonder how much hassle it would be to make linux APM-compliant?
>
>    For those who don't know, APM is the Intel/Microsoft name for Advanced
>    Power Management, a system that works to significantly increase battery
>    life in laptops.
>
> Does anyone know where to find the *specs* for APM?  Hopefully APM is
> speced out at a hardware level (send value X to port Y), instead of at
> the BIOS level (do INT 23 with AL set to 5).  If it's the second, it
> would be much harder to support APM than if it had a consistent hardware
> interface.  Unfortunately, knowing Intel/Microsoft, I doubt they did the
> right thing....
>
>                                                          - Ted

   I have already spent a few hours snooping about.  The only place I
found information so far is the excellent: "Comprehensive list of
interrupt calls" by Ralf Brown.  I also skimmed the "pink coloured
cover" edition of the "PC sourcebook" by Thom Hoggan (sp?) but didn't
find anything useful.  I'd be the first to admit that this cannot be
considered thorough research on the problem!  Anyway here is the little
I've found out so far...

---rbrown.txt---
Public Domain/Freeware/Shareware by Ralf Brown:

The MSDOS Interrupt List
        A comprehensive listing of more than 4070 functions available
        through interrupt calls on IBM PCs and compatibles.  Includes both
        documented and undocumented functions.
        Current version: INTER31A (release 31, 7/12/92)
                         INTER31B
                         INTER31C (conversion programs)
....
================

You can get your own copy from,

        wuarchive.wustl.edu:/mirrors/msdos/info/inter31{a,b,c}.zip

   There are some listings for power management routines.  (I have
attached the text for some of the core functions at the end of my
message ...)  The support is in the BIOS :(, fear not though - there are
provisions for 32 bit protected mode operation.

   The beast I am working on is a Zenith ZNOTE-320L and the INSTALLATION
CHECK interrupt claims that 32 bit protected mode support exists :) :)

    However, not knowing exactly how all the functions are supposed to
operate together I am "debugging" a live Laptop under DOS and making
notes and tweeks.  Quite a bit seems to be going on :)

NOTE: To any listening vendors that sell laptops: If I get power
management working on your laptop I just might rush out and buy one.  I
suspect many others would do likewise!  Offer to send me the specs if I
call :).  I have called Zenith, but I think that they misunderstood my
questions :( Persistence is a virtue!

   I've created a "laptops" thread.  I am guessing that there are number
of people interested in this line of development ...

   If anyone reading this happens to have a copy of the APM spec in a
machine readable form then I'd appreciate a copy.

Simon Cooper
Systems Coordinator
Laboratory for Visiometrics and Modeling
===============================================================================
Snail-mail: CAIP Center, Rutgers University, E-mail: scooper@vizlab.rutgers.edu
            Frelinghuysen Road, PO Box 1390
            Piscataway. NJ 08855-1390
Telephone: (+1) 908 932 0540                    Fax: (+1) 908 932 4775
===============================================================================

Reproduced without permission from Ralf Brown's interrupt list...

==========155300BX0000=======================
INT 15 - Advanced Power Management Specification - INSTALLATION CHECK
        AX = 5300h
        BX = 0000h (device ID of system BIOS)
Return: CF clear if successful
            AH = major version (BCD)
            AL = minor version (BCD)
            BX = 504Dh ("PM")
            CX = flags
                bit 0: 16-bit protected mode interface supported
                bit 1: 32-bit protected mode interface supported
                bit 2: CPU idle call reduces processor speed
                bit 3: BIOS power management disabled
                bits 4-7 reserved
        CF set on error
            AH = error code (86h) (see below)

Values for error code:
 01h    power management functionality disabled
 02h    interface connection already in effect
 03h    interface not connected
 04h    real-mode interface not connected
 05h    16-bit protected-mode interface already connected
 06h    16-bit protected-mode interface not supported
 07h    32-bit protected-mode interface already connected
 08h    32-bit protected-mode interface not supported
 09h    unrecognized device ID
 0Ah    invalid parameter value in CX
 0Bh-1Fh reserved for other interface and general errors
 20h-3Fh reserved for CPU errors
 40h-5Fh reserved for device errors
 60h    can't enter requested state
 61h-7Fh reserved for other system errors
 80h    no power management events pending
 81h-85h reserved for other power management event errors
 86h    APM not present
 87h-9Fh reserved for other power management event errors
==========155301BX0000=======================
INT 15 - Advanced Power Management Specification - CONNECT REAL-MODE INTERFACE
        AX = 5301h
        BX = 0000h (device ID of system BIOS)
Return: CF clear if successful
        CF set on error
            AH = error code (02h,09h) (see AX=5300h)
SeeAlso: AX=5302h,AX=5303h,AX=5304h
==========155302BX0000=======================
INT 15 R - Advanced Power Management Spec - CONNECT 16-BIT PROTMODE INTERFACE
        AX = 5302h
        BX = 0000h (device ID of system BIOS)
Return: CF clear if successful
            AX = real-mode segment base address of protected-mode 16-bit code
                segment
            BX = offset of entry point
            CX = real-mode segment base address of protected-mode 16-bit data
                segment
        CF set on error
            AH = error code (05h,06h,09h) (see AX=5300h)
Notes:  the caller must initialize two consecutive descriptors with the
          returned segment base addresses; these descriptors must be valid
          whenever the protected-mode interface is called, and will have
          their limits arbitrarily set to 64K.
        the protected mode interface is invoked by making a far call with the
          same register values as for INT 15; it must be invoked while CPL=0,
          the code segment descriptor must have a DPL of 0, the stack must be
          in a 16-bit segment and have enough room for BIOS use and possible
          interrupts, and the current I/O permission bit map must allow access
          to the I/O ports used for power management.
        functions 00h-03h are not available from protected mode
SeeAlso: AX=5301h,AX=5303h,AX=5304h
==========155303BX0000=======================
INT 15 - Advanced Power Management Spec - CONNECT 32-BIT PROTMODE INTERFACE
        AX = 5303h
        BX = 0000h (device ID of system BIOS)
Return: CF clear if successful
            AX = real-mode segment base address of protected-mode 32-bit code
                segment
            EBX = offset of entry point
            CX = real-mode segment base address of protected-mode 16-bit code
                segment
            DX = real-mode segment base address of protected-mode 16-bit data
                segment
        CF set on error
            AH = error code (07h,08h,09h) (see AX=5300h)
Notes:  the caller must initialize three consecutive descriptors with the
          returned segment base addresses for 32-bit code, 16-bit code, and
          16-bit data, respectively; these descriptors must be valid whenever
          the protected-mode interface is called, and will have their limits
          arbitrarily set to 64K.
        the protected mode interface is invoked by making a far call to the
          32-bit code segment with the same register values as for INT 15; it
          must be invoked while CPL=0, the code segment descriptor must have a
          DPL of 0, the stack must be in a 32-bit segment and have enough room
          for BIOS use and possible interrupts, and the current I/O permission
          bit map must allow access to the I/O ports used for power management.
        functions 00h-03h are not available from protected mode
SeeAlso: AX=5301h,AX=5302h,AX=5304h
==========155304BX0000=======================
INT 15 - Advanced Power Management Specification - DISCONNECT INTERFACE
        AX = 5304h
        BX = 0000h (device ID of system BIOS)
Return: CF clear if successful
        CF set on error
            AH = error code (03h,09h) (see AX=5300h)
SeeAlso: AX=5301h,AX=5302h,AX=5303h
==========155305=============================
INT 15 - Advanced Power Management Specification - CPU IDLE
        AX = 5305h
Return: after system leaves idle state
        CF clear
Notes:  call when the system is idle and should be suspended until the next
          system event or interrupt
        should not be called from within a hardware interrupt handler to avoid
          reentrance problems
        if an interrupt causes the system to resume normal processing, the
          interrupt may or may not have been handled when the BIOS returns
          from this call; thus, the caller should allow interrupts on return
        interrupt handlers may not retain control if the BIOS allows
          interrupts while in idle mode even if they are able to determine
          that they were called from idle mode
        the caller should issue this call continuously in a loop until it needs
          to perform some processing of its own
SeeAlso: AX=1000h,AX=5306h,INT 2F/AX=1680h
==========155306=============================
INT 15 - Advanced Power Management Specification - CPU BUSY
        AX = 5306h
Return: CF clear
Notes:  called to ensure that the system runs at full speed even on systems
          where the BIOS is unable to recognize increased activity (especially
          if interrupts are hooked by other programs and not chained to the
          BIOS)
        this call may be made even when the system is already running at full
          speed, but it will create unnecessary overhead
        should not be called from within a hardware interrupt handler to avoid
          reentrance problems
SeeAlso: AX=5305h
==========155307=============================
INT 15 - Advanced Power Management Specification - SET POWER STATE
        AX = 5307h
        BX = device ID (see below)
        CX = system state ID
            0000h ready (not supported for device ID 0001h)
            0001h stand-by
            0002h suspend
            0003h off (not supported for device ID 0001h)
            0004h-FFFFh reserved
Return: CF clear if successful
        CF set on error
            AH = error code (01h,09h,0Ah,60h) (see AX=5300h)
Note:   should not be called from within a hardware interrupt handler to avoid
          reentrance problems

Values for device IDs:
 0000h  system BIOS
 0001h  all devices for which the system BIOS manages power
 01xxh  display (01FFh for all attached display devices)
 02xxh  secondary storage (02FFh for all attached secondary storage devices)
 03xxh  parallel ports (03FFh for all attached parallel ports)
 04xxh  serial ports (04FFh for all attached serial ports)
 0500h-FFFFh reserved
==========155307BX0001=======================
INT 15 - Advanced Power Management Specification - SYSTEM STAND-BY
        AX = 5307h
        BX = 0001h
        CX = 0001h
Return: CF clear
Notes:  puts the entire system into stand-by mode; normally called in response
          to a System Stand-by Request notification after any necessary
          processing, but may also be invoked at the caller's discretion
        should not be called from within a hardware interrupt handler to avoid
          reentrance problems
        the stand-by state is typically exited on an interrupt
SeeAlso: AX=4280h,AX=5307h/BX=0001h/CX=0002h,AX=530Bh
==========155307BX0001=======================
INT 15 - Advanced Power Management Specification - SUSPEND SYSTEM
        AX = 5307h
        BX = 0001h
        CX = 0002h
Return: after system is resumed
        CF clear
Notes:  puts the entire system into a low-power suspended state; normally
          called in response to a Suspend System Request notification after
          any necessary processing, but may also be invoked at the caller's
          discretion
        should not be called from within a hardware interrupt handler to avoid
          reentrance problems
        the caller may need to update its date and time values because the
          system could have been suspended for a long period of time
SeeAlso: AX=5307h/BX=0001h/CX=0001h,AX=530Bh
==========155308BXFFFF=======================
INT 15 - Advanced Power Management Spec - ENABLE/DISABLE POWER MANAGEMENT
        AX = 5308h
        BX = FFFFh
        CX = new state
            0000h disabled
            0001h enabled
Return: CF clear if successful
        CF set on error
            AH = error code (01h,09h,0Ah) (see AX=5300h)
Notes:  when power management is disabled, the system BIOS will not
          automatically power down devices, enter stand-by or suspended mode,
          or perform any power-saving actions in response to AX=5305h calls
        should not be called from within a hardware interrupt handler to avoid
          reentrance problems
SeeAlso: AX=5309h
==========155309BXFFFF=======================
INT 15 - Advanced Power Management Specification - RESTORE POWER-ON DEFAULTS
        AX = 5309h
        BX = FFFFh
Return: CF clear if successful
        CF set on error
            AH = error code (09h) (see AX=5300h)
Note:   should not be called from within a hardware interrupt handler to avoid
          reentrance problems
SeeAlso: AX=5308h
==========15530ABX0001=======================
INT 15 - Advanced Power Management Specification - GET POWER STATUS
        AX = 530Ah
        BX = 0001h
Return: CF clear if successful
            BH = AC line status
                00h off-line
                01h on-line
                FFh unknown
                other reserved
            BL = battery status
                00h high
                01h low
                02h critical
                03h charging
                FFh unknown
                other reserved
            CL = remaining battery life
                00h-64h (0-100) percentage of full charge
                FFh unknown
        CF set on error
            AH = error code (09h) (see AX=5300h)
Note:   should not be called from within a hardware interrupt handler to avoid
          reentrance problems
==========15530B=============================
INT 15 - Advanced Power Management Specification - GET POWER MANAGEMENT EVENT
        AX = 530Bh
Return: CF clear if successful
            BX = event code
                0001h system stand-by request
                0002h system suspend request
                0003h normal resume system notification
                0004h critical resume system notification
                0005h battery low notification
        CF set on error
            AH = error code (03h,80h) (see AX=5300h)
Notes:  although power management events are often asynchronous, notification
          will not be made until polled via this call to permit software to
          only receive event notification when it is prepared to process
          power management events; since these events are not very time-
          critical, it should be sufficient to poll once or twice per second
        the critical resume notification is made after the system resumes
          from an emergency suspension; normally, the system BIOS only notifies
          its partner that it wishes to suspend and relies on the partner to
          actually request the suspension, but no notification is made on an
          emergency suspension
        should not be called from within a hardware interrupt handler to avoid
          reentrance problems
SeeAlso: AX=5307h,AX=5307h/BX=0001h/CX=0001h,AX=5307h/BX=0001h/CX=0002h