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|
BREAK <EMM control sector layout>
;
; The EMM control sector is a 1024 byte record which ALWAYS occupies the
; very first 1024 bytes of "extra" memory that needs to be managed. Its
; function is to provide a method to allocate available "extra" memory
; to programs which desire to use it and avoid program conflicts that
; would occur if two different programs attempted to use the same piece
; of "extra" memory.
;
;
; The EMM_CTRL structure defines the offsets into the 1024 byte control
; sector of the various fields. The EMM_REC structure defines a sub-structure
; contained within the EMM_CTRL structure which represents a particular
; piece of allocated "extra" memory (an allocation record).
;
; Layout of each EMM record.
EMM_REC STRUC
EMM_FLAGS DW 0
EMM_SYSTEM DW 0
EMM_BASE DD ? ; 24 bit address of start of region
EMM_KSIZE DW ? ; Size of region in kbytes
EMM_REC ENDS
; EMM_FLAGS Bits
EMM_ALLOC EQU 0000000000000001B ; Zero -> record is free
EMM_ISDRIVER EQU 0000000000000010B ; 1 -> driver is installed
; for this region
; EMM_SYSTEM Values
EMM_EMM EQU 0 ; Allocated to EMM
EMM_MSDOS EQU 1
EMM_XENIX EQU 2
EMM_APPLICATION EQU 3
; Layout of EMM control 1024 byte record
EMM_CTRL STRUC
EMM_VER DB 50 DUP(?)
EMM_TOTALK DW ? ; EXCLUDING the 1k of this record
EMM_AVAILK DW ? ; Amount of above NOT allocated
DB SIZE EMM_REC DUP(?) ; NULL (0th) RECORD
EMM_RECORD DB (1024 - 50 - 4 - 10 - (SIZE EMM_REC)) DUP(?)
; EMM_REC structures
EMM_TAIL_SIG DB 10 DUP(?)
EMM_CTRL ENDS
EMM_NUMREC EQU (1024 - 50 - 4 - 10 - (SIZE EMM_REC)) / (SIZE EMM_REC)
;
; The current initial (no "extra" memory allocated) EMM_CTRL sector is
;
; EMM_CONTROL LABEL BYTE
; DB "MICROSOFT EMM CTRL VERSION 1.00 CONTROL BLOCK "
; DW EXTMEM_TOTALK - 1
; DW EXTMEM_TOTALK - 1
; ; NULL 0th record
; DW EMM_ALLOC + EMM_ISDRIVER
; DW EMM_EMM
; DW EXTMEM_LOW + 1024
; DW EXTMEM_HIGH
; DW 0
; ;**
; DB 950 DUP(0)
; DB "ARRARRARRA"
;
; Where EXTMEM_LOW:EXTMEM_HIGH is the 32 bit address of the first byte
; of the EMM_CTRL sector (first byte of "extra" memory) and EXTMEM_TOTALK
; is the size in K of the available "extra" memory. One is subtracted
; from EXTMEM_TOTALK because the sizes in the EMM_CTRL record DO NOT
; include the 1k taken up by the EMM_CTRL sector.
;
; The reason for the existance of the NULL 0th record is to facilitate
; the computation of EMM_BASE for the first EMM_REC allocation record
; created.
;
; The EMM_REC structures CANNOT be sparse. In other words if one sets
; up a scan of the EMM_REC structures in the EMM_CTRL sector, as soon as
; an EMM_REC structure WITHOUT the EMM_ALLOC bit set in its flag word
; is encountered it is not necessary to scan further because it IS KNOWN
; that all of the EMM_REC structures after the first one with EMM_ALLOC
; clear also have EMM_ALLOC clear. What this means is that EMM_CTRL
; memory CANNOT BE deallocated. Once an EMM_REC structure has its
; EMM_ALLOC bit set, there is NO correct program operation which
; can clear the bit UNLESS it IS KNOWN that the next EMM_REC structure
; has its EMM_ALLOC bit clear or the EMM_REC structure is the last one.
;
;
; USING THE EMM_CTRL SECTOR:
;
; A program which wishes to use the EMM_CTRL sector to access "extra"
; memory should work as follows:
;
; Figure out how much memory you wish to allocate
;
; Figure out the location and size of the "extra" memory in the system
;
; IF (the first 1024 bytes of "extra" memory DO NOT contain a valid
; EMM_CTRL record determined by checking for the existence of the
; correct EMM_VER and EMM_TAIL_SIG strings)
; Write a correct initial EMM_CTRL sector to the first 1024
; bytes of extra memory. Be sure to fill in EMM_TOTALK,
; EMM_AVAILK and EMM_BASE in the 0th record.
;
; Set up a scan of the EMM_REC structures in the EMM_CTRL sector.
; NOTE: You can skip the NULL 0th record if you want since it has
; known value.
;
; FOR (i=0;i<EMM_NUMREC;i++)
; IF ( this EMM_REC has EMM_ALLOC clear)
; IF ( EMM_AVAILK < amount I want to alloc)
; ERROR insufficient memory
; EMM_AVAILK = EMM_AVAILK - amount I want to alloc
; EMM_FLAGS = EMM_ALLOC + EMM_ISDRIVER
; EMM_KSIZE = amount I want to alloc
; EMM_SYSTEM = appropriate value
; EMM_BASE = EMM_BASE of PREVIOUS EMM_REC +
; (1024 * EMM_KSIZE of PREVIOUS EMM_REC)
; break
; ELSE
; address next EMM_REC structure
;
; IF (i >= EMM_NUMREC)
; ERROR no free EMM_REC structures
;
;
; You can now see why we need that NUL 0th EMM_REC structure. In order to
; perform this step
;
; EMM_BASE = EMM_BASE of PREVIOUS EMM_REC +
; (1024 * EMM_KSIZE of PREVIOUS EMM_REC)
;
; when the very first EMM_REC is allocated we must have a "previous EMM_REC"
; structure to point at.
;
; The above code is rather simplistic in that all it does is do a simple
; allocation. The EMM_ISDRIVER bit allows us to do some more sophisticated
; things. In particular in the case of a RAMDrive type of program it is
; desirable to "re-find" the same RAMDrive area in "extra" memory when the
; system is re-booted. The EMM_ISDRIVER bit is used to help us do this.
;
; The EMM_ISDRIVER bit means "there is presently a piece of code in the
; system which is using this memory". If we find an EMM_REC structure
; which has its EMM_ALLOC bit set, but the EMM_ISDRIVER bit is clear
; it means that the piece of code that originally allocated
; the memory is gone and we may want to "re-find" this memory by
; setting the EMM_ISDRIVER bit again. A RAMDrive program would have
; slightly different code than the above:
;
; FOR (i=0;i<EMM_NUMREC;i++)
; IF ( this EMM_REC has EMM_ALLOC clear)
; IF ( EMM_AVAILK < amount I want to alloc)
; ERROR insufficient memory
; EMM_AVAILK = EMM_AVAILK - amount I want to alloc
; EMM_FLAGS = EMM_ALLOC + EMM_ISDRIVER
; EMM_KSIZE = amount I want to alloc
; EMM_SYSTEM = appropriate value
; EMM_BASE = EMM_BASE of PREVIOUS EMM_REC +
; (1024 * EMM_KSIZE of PREVIOUS EMM_REC)
; break
; ELSE
; IF ((EMM_SYSTEM == my value for EMM_SYSTEM) &&
; (EMM_ISDRIVER is clear))
; deal with differences between amount
; I want to allocate and EMM_KSIZE
; Set EMM_ISDRIVER
; EMM_BASE is the base address of this piece
; of memory and EMM_KSIZE is its size.
; break
; address next EMM_REC structure
;
; In this way we "re-find" memory that was previosly allocated (presumably
; by us, or a related program).
;
; NOTE THAT THIS USE OF EMM_ISDRIVER REQUIRES SOME MECHANISM FOR CLEARING
; EMM_ISDRIVER WHEN THE CODE OF INTEREST IS REMOVED FROM THE SYSTEM.
; In the case of a RAMDrive program the code is removed whenever the system
; is re-booted. For this reason a RAMDrive program will need code that is
; invoked whenever a system re-boot is detected. What this code does is
; scan the EMM_REC structures in the EMM_CTRL sector turning off the
; EMM_ISDRIVER bits:
;
; FOR (i=0;i<EMM_NUMREC;i++)
; IF ( this EMM_REC has EMM_ALLOC clear)
; break
; ELSE IF (EMM_SYSTEM == my value for EMM_SYSTEM)
; clear EMM_ISDRIVER bit
; address next EMM_REC
;
; Note that this code clears ALL of the ISDRIVER bits for EMM_SYSTEM
; values of a certain value. This means there is a GLOBAL piece of
; re-boot code for ALL of the programs using a particular EMM_SYSTEM
; value. An alternative is to have each EMM_CTRL user clear the
; EMM_ISDRIVER bits ONLY for those EMM_REC structures that it allocated.
; This requires that the program keep a record of which EMM_REC structures
; it is responsible for:
;
; FOR each of my EMM_REC structures
; INDEX this EMM_REC structure in the EMM_CTRL sector
; Clear EMM_ISDRIVER
;
; NOTE about this step:
;
; deal with differences between amount
; I want to allocate and EMM_KSIZE
;
; in the above code. There are a lot of options here depending on the desired
; behavior. If the NEXT EMM_REC structure has EMM_ALLOC clear, then it may
; be possible for me to grow or shrink the block I found by adjusting
; EMM_KSIZE and EMM_AVAILK. If the NEXT EMM_REC structure has EMM_ALLOC
; set, then I am forced to either adjust the amount I want to allocate
; to match EMM_KSIZE, or skip this EMM_REC without setting EMM_ISDRIVER.
;
;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
; for rom 1nt15 extended memory interface
emm_int equ 15h
emm_size equ 88h
emm_blkm equ 87h
;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
|
