dlls/krnl386.exe16/dosmem.c

   1 /*
   2  * DOS memory emulation
   3  *
   4  * Copyright 1995 Alexandre Julliard
   5  * Copyright 1996 Marcus Meissner
   6  *
   7  * This library is free software; you can redistribute it and/or
   8  * modify it under the terms of the GNU Lesser General Public
   9  * License as published by the Free Software Foundation; either
  10  * version 2.1 of the License, or (at your option) any later version.
  11  *
  12  * This library is distributed in the hope that it will be useful,
  13  * but WITHOUT ANY WARRANTY; without even the implied warranty of
  14  * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
  15  * Lesser General Public License for more details.
  16  *
  17  * You should have received a copy of the GNU Lesser General Public
  18  * License along with this library; if not, write to the Free Software
  19  * Foundation, Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301, USA
  20  */
  21
  22 #include "config.h"
  23 #include "wine/port.h"
  24
  25 #include <signal.h>
  26 #include <stdarg.h>
  27 #include <stdlib.h>
  28 #include <string.h>
  29 #include <sys/types.h>
  30 #ifdef HAVE_SYS_MMAN_H
  31 # include <sys/mman.h>
  32 #endif
  33
  34 #include "windef.h"
  35 #include "winbase.h"
  36 #include "excpt.h"
  37 #include "winternl.h"
  38 #include "wine/winbase16.h"
  39
  40 #include "kernel16_private.h"
  41 #include "dosexe.h"
  42 #include "wine/debug.h"
  43
  44 WINE_DEFAULT_DEBUG_CHANNEL(dosmem);
  45 WINE_DECLARE_DEBUG_CHANNEL(selector);
  46
  47 WORD DOSMEM_0000H;        /* segment at 0:0 */
  48 WORD DOSMEM_BiosDataSeg;  /* BIOS data segment at 0x40:0 */
  49 WORD DOSMEM_BiosSysSeg;   /* BIOS ROM segment at 0xf000:0 */
  50
  51 /* DOS memory highest address (including HMA) */
  52 #define DOSMEM_SIZE             0x110000
  53 #define DOSMEM_64KB             0x10000
  54
  55 /*
  56  * Memory Control Block (MCB) definition
  57  * FIXME: implement Allocation Strategy
  58  */
  59
  60 #define MCB_DUMP(mc) \
  61     TRACE ("MCB_DUMP base=%p type=%02xh psp=%04xh size=%04xh\n", mc, mc->type, mc->psp , mc->size )
  62
  63 #define MCB_NEXT(mc) \
  64     (MCB*) ((mc->type==MCB_TYPE_LAST) ? NULL : (char*)(mc) + ((mc->size + 1) << 4) )
  65
  66 /* FIXME: should we check more? */
  67 #define MCB_VALID(mc) \
  68     ((mc->type==MCB_TYPE_NORMAL) || (mc->type==MCB_TYPE_LAST))
  69
  70
  71 #define MCB_TYPE_NORMAL    0x4d
  72 #define MCB_TYPE_LAST      0x5a
  73
  74 #define MCB_PSP_DOS        0x0060
  75 #define MCB_PSP_FREE       0
  76
  77 #include "pshpack1.h"
  78 typedef struct {
  79     BYTE type;
  80     WORD psp;     /* segment of owner psp */
  81     WORD size;    /* in paragraphs */
  82     BYTE pad[3];
  83     BYTE name[8];
  84 } MCB;
  85 #include "poppack.h"
  86
  87 /*
  88 #define __DOSMEM_DEBUG__
  89  */
  90
  91 #define VM_STUB(x) (0x90CF00CD|(x<<8)) /* INT x; IRET; NOP */
  92 #define VM_STUB_SEGMENT 0xf000         /* BIOS segment */
  93
  94 /* FIXME: this should be moved to the LOL */
  95 static MCB* DOSMEM_root_block;
  96
  97 /* when looking at DOS and real mode memory, we activate in three different
  98  * modes, depending the situation.
  99  * 1/ By default (protected mode), the first MB of memory (actually 0x110000,
 100  *    when you also look at the HMA part) is always reserved, whatever you do.
 101  *    We allocated some PM selectors to this memory, even if this area is not
 102  *    committed at startup
 103  * 2/ if a program tries to use the memory through the selectors, we actually
 104  *    commit this memory, made of: BIOS segment, but also some system
 105  *    information, usually low in memory that we map for the circumstance also
 106  *    in the BIOS segment, so that we keep the low memory protected (for NULL
 107  *    pointer deref catching for example). In this case, we're still in PM
 108  *    mode, accessing part of the "physical" real mode memory. In fact, we don't
 109  *    map all the first meg, we keep 64k uncommitted to still catch NULL
 110  *    pointers dereference
 111  * 3/ if the process enters the real mode, then we (also) commit the full first
 112  *    MB of memory (and also initialize the DOS structures in it).
 113  */
 114
 115 /* DOS memory base (linear in process address space) */
 116 static char *DOSMEM_dosmem;
 117 static char *DOSMEM_sysmem;
 118 /* number of bytes protected from _dosmem. 0 when DOS memory is initialized,
 119  * 64k otherwise to trap NULL pointers deref */
 120 static DWORD DOSMEM_protect;
 121
 122 static LONG WINAPI dosmem_handler(EXCEPTION_POINTERS* except);
 123 static void *vectored_handler;
 124
 125 /***********************************************************************
 126  *           DOSMEM_FillIsrTable
 127  *
 128  * Fill the interrupt table with fake BIOS calls to BIOSSEG (0xf000).
 129  *
 130  * NOTES:
 131  * Linux normally only traps INTs performed from or destined to BIOSSEG
 132  * for us to handle, if the int_revectored table is empty. Filling the
 133  * interrupt table with calls to INT stubs in BIOSSEG allows DOS programs
 134  * to hook interrupts, as well as use their familiar retf tricks to call
 135  * them, AND let Wine handle any unhooked interrupts transparently.
 136  */
 137 static void DOSMEM_FillIsrTable(void)
 138 {
 139     SEGPTR *isr = (SEGPTR*)DOSMEM_sysmem;
 140     int x;
 141
 142     for (x=0; x<256; x++) isr[x]=MAKESEGPTR(VM_STUB_SEGMENT,x*4);
 143 }
 144
 145 static void DOSMEM_MakeIsrStubs(void)
 146 {
 147     DWORD *stub = (DWORD*)(DOSMEM_dosmem + (VM_STUB_SEGMENT << 4));
 148     int x;
 149
 150     for (x=0; x<256; x++) stub[x]=VM_STUB(x);
 151 }
 152
 153 BIOSDATA* DOSVM_BiosData(void)
 154 {
 155     return (BIOSDATA *)(DOSMEM_sysmem + 0x400);
 156 }
 157
 158 /**********************************************************************
 159  *          DOSMEM_GetTicksSinceMidnight
 160  *
 161  * Return number of clock ticks since midnight.
 162  */
 163 static DWORD DOSMEM_GetTicksSinceMidnight(void)
 164 {
 165     SYSTEMTIME time;
 166
 167     /* This should give us the (approximately) correct
 168      * 18.206 clock ticks per second since midnight.
 169      */
 170
 171     GetLocalTime( &time );
 172
 173     return (((time.wHour * 3600 + time.wMinute * 60 +
 174               time.wSecond) * 18206) / 1000) +
 175              (time.wMilliseconds * 1000 / 54927);
 176 }
 177
 178 /***********************************************************************
 179  *           DOSMEM_FillBiosSegments
 180  *
 181  * Fill the BIOS data segment with dummy values.
 182  */
 183 static void DOSMEM_FillBiosSegments(void)
 184 {
 185     BYTE *pBiosSys = (BYTE*)DOSMEM_dosmem + 0xf0000;
 186     BYTE *pBiosROMTable = pBiosSys+0xe6f5;
 187     BIOSDATA *pBiosData = DOSVM_BiosData();
 188     static const char bios_date[] = "13/01/99";
 189
 190       /* Clear all unused values */
 191     memset( pBiosData, 0, sizeof(*pBiosData) );
 192
 193     /* FIXME: should check the number of configured drives and ports */
 194     pBiosData->Com1Addr             = 0x3f8;
 195     pBiosData->Com2Addr             = 0x2f8;
 196     pBiosData->Lpt1Addr             = 0x378;
 197     pBiosData->Lpt2Addr             = 0x278;
 198     pBiosData->InstalledHardware    = 0x5463;
 199     pBiosData->MemSize              = 640;
 200     pBiosData->NextKbdCharPtr       = 0x1e;
 201     pBiosData->FirstKbdCharPtr      = 0x1e;
 202     pBiosData->VideoMode            = 3;
 203     pBiosData->VideoColumns         = 80;
 204     pBiosData->VideoPageSize        = 80 * 25 * 2;
 205     pBiosData->VideoPageStartAddr   = 0xb800;
 206     pBiosData->VideoCtrlAddr        = 0x3d4;
 207     pBiosData->Ticks                = DOSMEM_GetTicksSinceMidnight();
 208     pBiosData->NbHardDisks          = 2;
 209     pBiosData->KbdBufferStart       = 0x1e;
 210     pBiosData->KbdBufferEnd         = 0x3e;
 211     pBiosData->RowsOnScreenMinus1   = 24;
 212     pBiosData->BytesPerChar         = 0x10;
 213     pBiosData->ModeOptions          = 0x64;
 214     pBiosData->FeatureBitsSwitches  = 0xf9;
 215     pBiosData->VGASettings          = 0x51;
 216     pBiosData->DisplayCombination   = 0x08;
 217     pBiosData->DiskDataRate         = 0;
 218
 219     /* fill ROM configuration table (values from Award) */
 220     *(pBiosROMTable+0x0)        = 0x08; /* number of bytes following LO */
 221     *(pBiosROMTable+0x1)        = 0x00; /* number of bytes following HI */
 222     *(pBiosROMTable+0x2)        = 0xfc; /* model */
 223     *(pBiosROMTable+0x3)        = 0x01; /* submodel */
 224     *(pBiosROMTable+0x4)        = 0x00; /* BIOS revision */
 225     *(pBiosROMTable+0x5)        = 0x74; /* feature byte 1 */
 226     *(pBiosROMTable+0x6)        = 0x00; /* feature byte 2 */
 227     *(pBiosROMTable+0x7)        = 0x00; /* feature byte 3 */
 228     *(pBiosROMTable+0x8)        = 0x00; /* feature byte 4 */
 229     *(pBiosROMTable+0x9)        = 0x00; /* feature byte 5 */
 230
 231     /* BIOS date string */
 232     memcpy(pBiosSys+0xfff5, bios_date, sizeof bios_date);
 233
 234     /* BIOS ID */
 235     *(pBiosSys+0xfffe) = 0xfc;
 236
 237     /* Reboot vector (f000:fff0 or ffff:0000) */
 238     *(DWORD*)(pBiosSys + 0xfff0) = VM_STUB(0x19);
 239 }
 240
 241 /***********************************************************************
 242  *           BiosTick
 243  *
 244  * Increment the BIOS tick counter. Called by timer signal handler.
 245  */
 246 static void CALLBACK BiosTick( LPVOID arg, DWORD low, DWORD high )
 247 {
 248     BIOSDATA *pBiosData = arg;
 249     pBiosData->Ticks++;
 250 }
 251
 252 /***********************************************************************
 253  *           timer_thread
 254  */
 255 static DWORD CALLBACK timer_thread( void *arg )
 256 {
 257     LARGE_INTEGER when;
 258     HANDLE timer;
 259
 260     if (!(timer = CreateWaitableTimerA( NULL, FALSE, NULL ))) return 0;
 261
 262     when.u.LowPart = when.u.HighPart = 0;
 263     SetWaitableTimer( timer, &when, 55 /* actually 54.925 */, BiosTick, arg, FALSE );
 264     for (;;) SleepEx( INFINITE, TRUE );
 265 }
 266
 267 /***********************************************************************
 268  *           DOSVM_start_bios_timer
 269  *
 270  * Start the BIOS ticks timer when the app accesses selector 0x40.
 271  */
 272 void DOSVM_start_bios_timer(void)
 273 {
 274     static LONG running;
 275
 276     if (!InterlockedExchange( &running, 1 ))
 277         CloseHandle( CreateThread( NULL, 0, timer_thread, DOSVM_BiosData(), 0, NULL ));
 278 }
 279
 280 /***********************************************************************
 281  *           DOSMEM_Collapse
 282  *
 283  * Helper function for internal use only.
 284  * Attach all following free blocks to this one, even if this one is not free.
 285  */
 286 static void DOSMEM_Collapse( MCB* mcb )
 287 {
 288     MCB* next = MCB_NEXT( mcb );
 289
 290     while (next && next->psp == MCB_PSP_FREE)
 291     {
 292         mcb->size = mcb->size + next->size + 1;
 293         mcb->type = next->type;    /* make sure keeping MCB_TYPE_LAST */
 294         next = MCB_NEXT( next );
 295     }
 296 }
 297
 298 /******************************************************************
 299  *              DOSMEM_InitDosMemory
 300  */
 301 BOOL DOSMEM_InitDosMemory(void)
 302 {
 303     static BOOL done;
 304     static HANDLE hRunOnce;
 305
 306     if (done) return TRUE;
 307
 308     /* FIXME: this isn't 100% thread safe, as we won't catch accesses while initializing */
 309     if (hRunOnce == 0)
 310     {
 311         HANDLE hEvent = CreateEventW( NULL, TRUE, FALSE, NULL );
 312         if (InterlockedCompareExchangePointer( &hRunOnce, hEvent, 0 ) == 0)
 313         {
 314             BOOL ret;
 315             DWORD reserve;
 316
 317             /* ok, we're the winning thread */
 318             if (!(ret = VirtualProtect( DOSMEM_dosmem + DOSMEM_protect,
 319                                         DOSMEM_SIZE - DOSMEM_protect,
 320                                         PAGE_READWRITE, NULL )))
 321                 ERR("Cannot load access low 1Mb, DOS subsystem unavailable\n");
 322             RemoveVectoredExceptionHandler( vectored_handler );
 323
 324             /*
 325              * Reserve either:
 326              * - lowest 64k for NULL pointer catching (Win16)
 327              * - lowest 1k for interrupt handlers and
 328              *   another 0.5k for BIOS, DOS and intra-application
 329              *   areas (DOS)
 330              */
 331             if (DOSMEM_dosmem != DOSMEM_sysmem)
 332                 reserve = 0x10000; /* 64k */
 333             else
 334                 reserve = 0x600; /* 1.5k */
 335
 336             /*
 337              * Set DOS memory base and initialize conventional memory.
 338              */
 339             DOSMEM_FillBiosSegments();
 340             DOSMEM_FillIsrTable();
 341
 342             /* align root block to paragraph */
 343             DOSMEM_root_block = (MCB*)(DOSMEM_dosmem + reserve);
 344             DOSMEM_root_block->type = MCB_TYPE_LAST;
 345             DOSMEM_root_block->psp = MCB_PSP_FREE;
 346             DOSMEM_root_block->size = (DOSMEM_dosmem + 0x9fffc  - ((char*)DOSMEM_root_block)) >> 4;
 347
 348             TRACE("DOS conventional memory initialized, %d bytes free.\n",
 349                   DOSMEM_Available());
 350
 351             DOSVM_InitSegments();
 352
 353             SetEvent( hRunOnce );
 354             done = TRUE;
 355             return ret;
 356         }
 357         /* someone beat us here... */
 358         CloseHandle( hEvent );
 359     }
 360
 361     /* and wait for the winner to have finished */
 362     WaitForSingleObject( hRunOnce, INFINITE );
 363     return TRUE;
 364 }
 365
 366 /******************************************************************
 367  *              dosmem_handler
 368  *
 369  * Handler to catch access to our 1MB address space reserved for real memory
 370  */
 371 static LONG WINAPI dosmem_handler(EXCEPTION_POINTERS* except)
 372 {
 373     if (except->ExceptionRecord->ExceptionCode == EXCEPTION_ACCESS_VIOLATION)
 374     {
 375         char *addr = (char *)except->ExceptionRecord->ExceptionInformation[1];
 376         if (addr >= DOSMEM_dosmem + DOSMEM_protect && addr < DOSMEM_dosmem + DOSMEM_SIZE)
 377         {
 378             if (DOSMEM_InitDosMemory()) return EXCEPTION_CONTINUE_EXECUTION;
 379         }
 380     }
 381     return EXCEPTION_CONTINUE_SEARCH;
 382 }
 383
 384 /***********************************************************************
 385  *           DOSMEM_Init
 386  *
 387  * Create the dos memory segments, and store them into the KERNEL
 388  * exported values.
 389  */
 390 BOOL DOSMEM_Init(void)
 391 {
 392     void *addr = (void *)1;
 393     SIZE_T size = DOSMEM_SIZE - 1;
 394
 395     if (NtAllocateVirtualMemory( GetCurrentProcess(), &addr, 0, &size,
 396                                  MEM_RESERVE | MEM_COMMIT, PAGE_NOACCESS ))
 397     {
 398         ERR( "Cannot allocate DOS memory\n" );
 399         ExitProcess(1);
 400     }
 401
 402     if (addr <= (void *)DOSMEM_64KB)
 403     {
 404         DOSMEM_dosmem = 0;
 405         DOSMEM_protect = DOSMEM_64KB;
 406         DOSMEM_sysmem = (char *)0xf0000;  /* store sysmem in high addresses for now */
 407     }
 408     else
 409     {
 410         WARN( "First megabyte not available for DOS address space.\n" );
 411         DOSMEM_dosmem = addr;
 412         DOSMEM_protect = 0;
 413         DOSMEM_sysmem = DOSMEM_dosmem;
 414     }
 415
 416     vectored_handler = AddVectoredExceptionHandler(FALSE, dosmem_handler);
 417     DOSMEM_0000H = GLOBAL_CreateBlock( GMEM_FIXED, DOSMEM_sysmem,
 418                                        DOSMEM_64KB, 0, WINE_LDT_FLAGS_DATA );
 419     DOSMEM_BiosDataSeg = GLOBAL_CreateBlock( GMEM_FIXED, DOSMEM_sysmem + 0x400,
 420                                              0x100, 0, WINE_LDT_FLAGS_DATA );
 421     DOSMEM_BiosSysSeg = GLOBAL_CreateBlock( GMEM_FIXED, DOSMEM_dosmem + 0xf0000,
 422                                             DOSMEM_64KB, 0, WINE_LDT_FLAGS_DATA );
 423
 424     return TRUE;
 425 }
 426
 427 /***********************************************************************
 428  *           DOSMEM_MapLinearToDos
 429  *
 430  * Linear address to the DOS address space.
 431  */
 432 UINT DOSMEM_MapLinearToDos(LPVOID ptr)
 433 {
 434     if (((char*)ptr >= DOSMEM_dosmem) &&
 435         ((char*)ptr < DOSMEM_dosmem + DOSMEM_SIZE))
 436           return (char *)ptr - DOSMEM_dosmem;
 437     return (UINT)ptr;
 438 }
 439
 440
 441 /***********************************************************************
 442  *           DOSMEM_MapDosToLinear
 443  *
 444  * DOS linear address to the linear address space.
 445  */
 446 LPVOID DOSMEM_MapDosToLinear(UINT ptr)
 447 {
 448     if (ptr < DOSMEM_SIZE) return DOSMEM_dosmem + ptr;
 449     return (LPVOID)ptr;
 450 }
 451
 452
 453 /***********************************************************************
 454  *           DOSMEM_MapRealToLinear
 455  *
 456  * Real mode DOS address into a linear pointer
 457  */
 458 LPVOID DOSMEM_MapRealToLinear(DWORD x)
 459 {
 460    LPVOID       lin;
 461
 462    lin = DOSMEM_dosmem + HIWORD(x) * 16 + LOWORD(x);
 463    TRACE_(selector)("(0x%08x) returns %p.\n", x, lin );
 464    return lin;
 465 }
 466
 467 /***********************************************************************
 468  *           DOSMEM_AllocBlock
 469  *
 470  * Carve a chunk of the DOS memory block (without selector).
 471  */
 472 LPVOID DOSMEM_AllocBlock(UINT size, UINT16* pseg)
 473 {
 474     MCB *curr;
 475     MCB *next = NULL;
 476     WORD psp;
 477
 478     DOSMEM_InitDosMemory();
 479
 480     curr = DOSMEM_root_block;
 481     if (!(psp = DOSVM_psp)) psp = MCB_PSP_DOS;
 482
 483     if (pseg) *pseg = 0;
 484
 485     TRACE( "(%04xh)\n", size );
 486
 487     /* round up to paragraph */
 488     size = (size + 15) >> 4;
 489
 490 #ifdef __DOSMEM_DEBUG__
 491     DOSMEM_Available();     /* checks the whole MCB list */
 492 #endif
 493
 494     /* loop over all MCB and search the next large enough MCB */
 495     while (curr)
 496     {
 497         if (!MCB_VALID (curr))
 498         {
 499             ERR( "MCB List Corrupt\n" );
 500             MCB_DUMP( curr );
 501             return NULL;
 502         }
 503         if (curr->psp == MCB_PSP_FREE)
 504         {
 505             DOSMEM_Collapse( curr );
 506             /* is it large enough (one paragraph for the MCB)? */
 507             if (curr->size >= size)
 508             {
 509                 if (curr->size > size)
 510                 {
 511                     /* split curr */
 512                     next = (MCB *) ((char*) curr + ((size+1) << 4));
 513                     next->psp = MCB_PSP_FREE;
 514                     next->size = curr->size - (size+1);
 515                     next->type = curr->type;
 516                     curr->type = MCB_TYPE_NORMAL;
 517                     curr->size = size;
 518                 }
 519                 /* curr is the found block */
 520                 curr->psp = psp;
 521                 if( pseg ) *pseg = (((char*)curr) + 16 - DOSMEM_dosmem) >> 4;
 522                 return (LPVOID) ((char*)curr + 16);
 523             }
 524         }
 525         curr = MCB_NEXT(curr);
 526     }
 527     return NULL;
 528 }
 529
 530 /***********************************************************************
 531  *           DOSMEM_FreeBlock
 532  */
 533 BOOL DOSMEM_FreeBlock(void* ptr)
 534 {
 535     MCB* mcb = (MCB*) ((char*)ptr - 16);
 536
 537     TRACE( "(%p)\n", ptr );
 538
 539 #ifdef __DOSMEM_DEBUG__
 540     DOSMEM_Available();
 541 #endif
 542
 543     if (!MCB_VALID (mcb))
 544     {
 545         ERR( "MCB invalid\n" );
 546         MCB_DUMP( mcb );
 547         return FALSE;
 548     }
 549
 550     mcb->psp = MCB_PSP_FREE;
 551     DOSMEM_Collapse( mcb );
 552     return TRUE;
 553 }
 554
 555 /***********************************************************************
 556  *           DOSMEM_ResizeBlock
 557  *
 558  * Resize DOS memory block in place. Returns block size or -1 on error.
 559  *
 560  * If exact is TRUE, returned value is either old or requested block
 561  * size. If exact is FALSE, block is expanded even if there is not
 562  * enough space for full requested block size.
 563  *
 564  * TODO: return also biggest block size
 565  */
 566 UINT DOSMEM_ResizeBlock(void *ptr, UINT size, BOOL exact)
 567 {
 568     MCB* mcb = (MCB*) ((char*)ptr - 16);
 569     MCB* next;
 570
 571     TRACE( "(%p,%04xh,%s)\n", ptr, size, exact ? "TRUE" : "FALSE" );
 572
 573     /* round up to paragraph */
 574     size = (size + 15) >> 4;
 575
 576 #ifdef __DOSMEM_DEBUG__
 577     DOSMEM_Available();
 578 #endif
 579
 580     if (!MCB_VALID (mcb))
 581     {
 582         ERR( "MCB invalid\n" );
 583         MCB_DUMP( mcb );
 584         return -1;
 585     }
 586
 587     /* resize needed? */
 588     if (mcb->size == size)
 589         return size << 4;
 590
 591     /* collapse free blocks */
 592     DOSMEM_Collapse( mcb );
 593
 594     /* shrink mcb ? */
 595     if (mcb->size > size)
 596     {
 597         next = (MCB *) ((char*)mcb + ((size+1) << 4));
 598         next->type = mcb->type;
 599         next->psp = MCB_PSP_FREE;
 600         next->size = mcb->size - (size+1);
 601         mcb->type = MCB_TYPE_NORMAL;
 602         mcb->size = size;
 603         return size << 4;
 604     }
 605
 606     if (!exact)
 607     {
 608         return mcb->size << 4;
 609     }
 610
 611     return -1;
 612 }
 613
 614 /***********************************************************************
 615  *           DOSMEM_Available
 616  */
 617 UINT DOSMEM_Available(void)
 618 {
 619     UINT  available = 0;
 620     UINT  total = 0;
 621     MCB *curr = DOSMEM_root_block;
 622     /* loop over all MCB and search the largest free MCB */
 623     while (curr)
 624     {
 625 #ifdef __DOSMEM_DEBUG__
 626         MCB_DUMP( curr );
 627 #endif
 628         if (!MCB_VALID (curr))
 629         {
 630             ERR( "MCB List Corrupt\n" );
 631             MCB_DUMP( curr );
 632             return 0;
 633         }
 634         if (curr->psp == MCB_PSP_FREE &&
 635             curr->size > available )
 636             available = curr->size;
 637
 638         total += curr->size + 1;
 639         curr = MCB_NEXT( curr );
 640     }
 641     TRACE( " %04xh of %04xh paragraphs available\n", available, total );
 642     return available << 4;
 643 }
 644
 645 /******************************************************************
 646  *              DOSMEM_MapDosLayout
 647  *
 648  * Initialize the first MB of memory to look like a real DOS setup
 649  */
 650 BOOL DOSMEM_MapDosLayout(void)
 651 {
 652     static BOOL already_mapped;
 653
 654     if (!already_mapped)
 655     {
 656         if (DOSMEM_dosmem || !VirtualProtect( NULL, DOSMEM_SIZE, PAGE_EXECUTE_READWRITE, NULL ))
 657         {
 658             ERR( "Need full access to the first megabyte for DOS mode\n" );
 659             ExitProcess(1);
 660         }
 661         /* copy the BIOS and ISR area down */
 662         memcpy( DOSMEM_dosmem, DOSMEM_sysmem, 0x400 + 0x100 );
 663         DOSMEM_sysmem = DOSMEM_dosmem;
 664         SetSelectorBase( DOSMEM_0000H, 0 );
 665         SetSelectorBase( DOSMEM_BiosDataSeg, 0x400 );
 666         /* we may now need the actual interrupt stubs, and since we've just moved the
 667          * interrupt vector table away, we can fill the area with stubs instead... */
 668         DOSMEM_MakeIsrStubs();
 669         already_mapped = TRUE;
 670     }
 671     return TRUE;
 672 }