QDOS Internals

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Common & User Heaps - Worked Example

This file shows the use of the 4 memory allocation and de-allocation traps to allocate and release areas of common and user heaps.

Traps used are :

The example code below carries out the following tasks :

  • Allocate a 64KB area of common heap to use as a user heap.
  • Link this area into a new user heap.
  • Allocate a 200 byte space in the new user heap.
  • Use it for some purpose or other.
  • De-allocate the 200 byes, returning them to free space in the user heap.
  • De-allocate the common heap area when done.
; First, allocate a 64Kb common heap area :
start   moveq   #mt_alchp,d0      ; Trap code
        move.l  #65536,d1         ; 64Kb required
        moveq   #-1,d2            ; This job is the owner of the area allocated
        trap    #1                ; Allocate the space
        tst.l   d0                ; Did it work ?
        beq.s   HeapOk            ; Yes
        <handle out of memory errors here>
; We have a common heap, convert it to a user heap :
HeapOk  moveq   #mt_lnkfr,d0      ; Trap code
        suba.l  a6,a0             ; We need A0 to be A6 relative
        lea     MyHeap,a1         ; Pointer to heap header
        move.l  0,(a1)            ; Indicate this is a new heap
        suba.l  a6,a1             ; This needs to be relative A6 as well
        trap    #1                ; That should do it (No errors occur here)

; We now have a large 64Kb user heap allocated. lets use 200 bytes :
        moveq   #mt_alloc,d0      ; Trap code
        move.l  #200,d1           ; I need a 200 bytes area of user heap
        lea     MyHeap,a0         ; MyHeap holds the address where my heap lives
        suba.l  a6,a0             ; Ensure that MyHeap is A6 relative        
        trap    #1                ; Allocate my 200 bytes, or fail ERR_OM
        tst.l   d0                ; Did it work ?
        beq.s   UseHeap           ; Yes
        <handle out of user heap memory errors here>
; Now we have allocated some user heap, use it somehow
UseHeap adda.l  a6,a0             ; The address we get back is relative A6, so absolute it
        <do something with our 200 bytes here from address (A0) onwards>

At this point in our program we can make use to the 200 bytes allocated in our user heap, allocate more chunks of space from our 64 KB allocation and so on. When we are done with the usage of the space, we can either end the job which will free up any resources allocated, or we can be tidy and do it ourselves, as follows.

; We are finished with our 200 bytes, so de-allocate it
FrUser  moveq   #mt_lnkfr,d0      ; Trap code
        move.l  0(a0),d1          ; Size - assumes A0 has not changed during usage above
        suba.l  a6,a0             ; A0 has to be relative a6
        lea     MyHeap,a1         ; Pointer to top of heap
        suba.l  a6,a1             ; Which has also to be relative A6
        trap    #1                ; Deallocate the 200 byte area

; Finally, we are all done, so de-allocate the common heap too
FrCmn   lea    MyHeap,a0          ; Address of the area live here
        move.l (a0),a0            ; Get the address of the area
        trap   #1                 ; Return 64Kb to the common heap
; Do other stuff here ......

; Storage address for the pointer to the top of our user heap
MyHeap  ds.l    1                 ; One long word of storage for a user heap head pointer
qdosmsq/traps/trap_1/heaps.txt · Last modified: 2009/01/30 11:33 by norman