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; Thus, the specific allocator is quite easy to write:

; allocate one page, split into blocks, maintain the single-linked

; list of all free blocks in each page.

; Note: size must be a multiple of required alignment.

; Data for one pool: dd pointer to the first page, MUTEX lock.

; Allocator for fixed-size blocks: allocate a block.

; [ebx-4] = pointer to the first page, ebx = pointer to MUTEX structure.

proc usb_allocate_common

        push    edi     ; save used register to be stdcall

virtual at esp

        dd      ?       ; saved edi

        dd      ?       ; return address

.size   dd      ?

end virtual

; 1. Take the lock.

        mov     ecx, ebx

        call    mutex_lock

; 2. Find the first allocated page with a free block, if any.

; 2a. Initialize for the loop.

        mov     edx, ebx

.pageloop:

; 2b. Get the next page, keeping the current in eax.

        mov     eax, edx

        mov     edx, [edx-4]

; 2c. If there is no next page, we're out of luck; go to 4.

        test    edx, edx

        jz      .newpage

        add     edx, 0x1000

@@:

; 2d. Get the pointer to the first free block on this page.

; If there is no free block, continue to 2b.

        mov     eax, [edx-8]

        test    eax, eax

        jz      .pageloop

; 2e. Get the pointer to the next free block.

        mov     ecx, [eax]

; 2f. Update the pointer to the first free block from eax to ecx.

; Normally [edx-8] still contains eax, if so, atomically set it to ecx

; and proceed to 3.

; However, the price of simplicity of usb_free_common (in particular, it

; doesn't take the lock) is that [edx-8] could (rarely) be changed while

; we processed steps 2d+2e. If so, return to 2d and retry.

        lock cmpxchg [edx-8], ecx

        jnz     @b

.return:

        push    eax

        mov     ecx, ebx

        call    mutex_unlock

        pop     eax

        pop     edi     ; restore used register to be stdcall

        ret     4

.newpage:

; 4. Allocate a new page.

        push    eax

        stdcall kernel_alloc, 0x1000

        pop     edx

; If failed, say something to the debug board and return zero.

        test    eax, eax

        jz      .nomemory

; 5. Add the new page to the tail of list of allocated pages.

        mov     [edx-4], eax

; 6. Initialize two service dwords in the end of page:

; first free block is (start of page) + (block size)

; (we will return first block at (start of page), so consider it allocated),

; no next page.

        mov     edx, eax

        lea     edi, [eax+0x1000-8]

        add     edx, [.size]

        mov     [edi], edx

        and     dword [edi+4], 0

; 7. All blocks starting from edx are free; join them in a single-linked list.

@@:

        mov     ecx, edx

        add     edx, [.size]

        mov     [ecx], edx

        cmp     edx, edi

        jbe     @b

        sub     ecx, [.size]

        and     dword [ecx], 0

; 8. Return (start of page).

        jmp     .return

.nomemory:

        dbgstr 'no memory for USB descriptor'

        xor     eax, eax

        jmp     .return

endp

; Allocator for fixed-size blocks: free a block.

proc usb_free_common

        push    ecx edx

virtual at esp

        rd      2       ; saved registers

        dd      ?       ; return address

.block  dd      ?

end virtual

; Insert the given block to the head of free blocks in this page.

        mov     ecx, [.block]

        mov     edx, ecx

        or      edx, 0xFFF

@@:

        mov     eax, [edx+1-8]

        mov     [ecx], eax

        lock cmpxchg [edx+1-8], ecx

        jnz     @b

        pop     edx ecx

        ret     4

endp