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
 124 /***********************************************************************
 125  *           DOSMEM_FillIsrTable
 126  *
 127  * Fill the interrupt table with fake BIOS calls to BIOSSEG (0xf000).
 128  *
 129  * NOTES:
 130  * Linux normally only traps INTs performed from or destined to BIOSSEG
 131  * for us to handle, if the int_revectored table is empty. Filling the
 132  * interrupt table with calls to INT stubs in BIOSSEG allows DOS programs
 133  * to hook interrupts, as well as use their familiar retf tricks to call
 134  * them, AND let Wine handle any unhooked interrupts transparently.
 135  */
 136 static void DOSMEM_FillIsrTable(void)
 137 {
 138     SEGPTR *isr = (SEGPTR*)DOSMEM_sysmem;
 139     int x;
 140
 141     for (x=0; x<256; x++) isr[x]=MAKESEGPTR(VM_STUB_SEGMENT,x*4);
 142 }
 143
 144 static void DOSMEM_MakeIsrStubs(void)
 145 {
 146     DWORD *stub = (DWORD*)(DOSMEM_dosmem + (VM_STUB_SEGMENT << 4));
 147     int x;
 148
 149     for (x=0; x<256; x++) stub[x]=VM_STUB(x);
 150 }
 151
 152 BIOSDATA* DOSVM_BiosData(void)
 153 {
 154     return (BIOSDATA *)(DOSMEM_sysmem + 0x400);
 155 }
 156
 157 /**********************************************************************
 158  *          DOSMEM_GetTicksSinceMidnight
 159  *
 160  * Return number of clock ticks since midnight.
 161  */
 162 static DWORD DOSMEM_GetTicksSinceMidnight(void)
 163 {
 164     SYSTEMTIME time;
 165
 166     /* This should give us the (approximately) correct
 167      * 18.206 clock ticks per second since midnight.
 168      */
 169
 170     GetLocalTime( &time );
 171
 172     return (((time.wHour * 3600 + time.wMinute * 60 +
 173               time.wSecond) * 18206) / 1000) +
 174              (time.wMilliseconds * 1000 / 54927);
 175 }
 176
 177 /***********************************************************************
 178  *           DOSMEM_FillBiosSegments
 179  *
 180  * Fill the BIOS data segment with dummy values.
 181  */
 182 static void DOSMEM_FillBiosSegments(void)
 183 {
 184     BYTE *pBiosSys = (BYTE*)DOSMEM_dosmem + 0xf0000;
 185     BYTE *pBiosROMTable = pBiosSys+0xe6f5;
 186     BIOSDATA *pBiosData = DOSVM_BiosData();
 187     static const char bios_date[] = "13/01/99";
 188
 189       /* Clear all unused values */
 190     memset( pBiosData, 0, sizeof(*pBiosData) );
 191
 192     /* FIXME: should check the number of configured drives and ports */
 193     pBiosData->Com1Addr             = 0x3f8;
 194     pBiosData->Com2Addr             = 0x2f8;
 195     pBiosData->Lpt1Addr             = 0x378;
 196     pBiosData->Lpt2Addr             = 0x278;
 197     pBiosData->InstalledHardware    = 0x5463;
 198     pBiosData->MemSize              = 640;
 199     pBiosData->NextKbdCharPtr       = 0x1e;
 200     pBiosData->FirstKbdCharPtr      = 0x1e;
 201     pBiosData->VideoMode            = 3;
 202     pBiosData->VideoColumns         = 80;
 203     pBiosData->VideoPageSize        = 80 * 25 * 2;
 204     pBiosData->VideoPageStartAddr   = 0xb800;
 205     pBiosData->VideoCtrlAddr        = 0x3d4;
 206     pBiosData->Ticks                = DOSMEM_GetTicksSinceMidnight();
 207     pBiosData->NbHardDisks          = 2;
 208     pBiosData->KbdBufferStart       = 0x1e;
 209     pBiosData->KbdBufferEnd         = 0x3e;
 210     pBiosData->RowsOnScreenMinus1   = 24;
 211     pBiosData->BytesPerChar         = 0x10;
 212     pBiosData->ModeOptions          = 0x64;
 213     pBiosData->FeatureBitsSwitches  = 0xf9;
 214     pBiosData->VGASettings          = 0x51;
 215     pBiosData->DisplayCombination   = 0x08;
 216     pBiosData->DiskDataRate         = 0;
 217
 218     /* fill ROM configuration table (values from Award) */
 219     *(pBiosROMTable+0x0)        = 0x08; /* number of bytes following LO */
 220     *(pBiosROMTable+0x1)        = 0x00; /* number of bytes following HI */
 221     *(pBiosROMTable+0x2)        = 0xfc; /* model */
 222     *(pBiosROMTable+0x3)        = 0x01; /* submodel */
 223     *(pBiosROMTable+0x4)        = 0x00; /* BIOS revision */
 224     *(pBiosROMTable+0x5)        = 0x74; /* feature byte 1 */
 225     *(pBiosROMTable+0x6)        = 0x00; /* feature byte 2 */
 226     *(pBiosROMTable+0x7)        = 0x00; /* feature byte 3 */
 227     *(pBiosROMTable+0x8)        = 0x00; /* feature byte 4 */
 228     *(pBiosROMTable+0x9)        = 0x00; /* feature byte 5 */
 229
 230     /* BIOS date string */
 231     memcpy(pBiosSys+0xfff5, bios_date, sizeof bios_date);
 232
 233     /* BIOS ID */
 234     *(pBiosSys+0xfffe) = 0xfc;
 235
 236     /* Reboot vector (f000:fff0 or ffff:0000) */
 237     *(DWORD*)(pBiosSys + 0xfff0) = VM_STUB(0x19);
 238 }
 239
 240 /***********************************************************************
 241  *           BiosTick
 242  *
 243  * Increment the BIOS tick counter. Called by timer signal handler.
 244  */
 245 static void CALLBACK BiosTick( LPVOID arg, DWORD low, DWORD high )
 246 {
 247     BIOSDATA *pBiosData = arg;
 248     pBiosData->Ticks++;
 249 }
 250
 251 /***********************************************************************
 252  *           timer_thread
 253  */
 254 static DWORD CALLBACK timer_thread( void *arg )
 255 {
 256     LARGE_INTEGER when;
 257     HANDLE timer;
 258
 259     if (!(timer = CreateWaitableTimerA( NULL, FALSE, NULL ))) return 0;
 260
 261     when.u.LowPart = when.u.HighPart = 0;
 262     SetWaitableTimer( timer, &when, 55 /* actually 54.925 */, BiosTick, arg, FALSE );
 263     for (;;) SleepEx( INFINITE, TRUE );
 264 }
 265
 266 /***********************************************************************
 267  *           DOSVM_start_bios_timer
 268  *
 269  * Start the BIOS ticks timer when the app accesses selector 0x40.
 270  */
 271 void DOSVM_start_bios_timer(void)
 272 {
 273     static LONG running;
 274
 275     if (!InterlockedExchange( &running, 1 ))
 276         CloseHandle( CreateThread( NULL, 0, timer_thread, DOSVM_BiosData(), 0, NULL ));
 277 }
 278
 279 /***********************************************************************
 280  *           DOSMEM_Collapse
 281  *
 282  * Helper function for internal use only.
 283  * Attach all following free blocks to this one, even if this one is not free.
 284  */
 285 static void DOSMEM_Collapse( MCB* mcb )
 286 {
 287     MCB* next = MCB_NEXT( mcb );
 288
 289     while (next && next->psp == MCB_PSP_FREE)
 290     {
 291         mcb->size = mcb->size + next->size + 1;
 292         mcb->type = next->type;    /* make sure keeping MCB_TYPE_LAST */
 293         next = MCB_NEXT( next );
 294     }
 295 }
 296
 297 /******************************************************************
 298  *              DOSMEM_InitDosMemory
 299  */
 300 BOOL DOSMEM_InitDosMemory(void)
 301 {
 302     static int done;
 303     static HANDLE hRunOnce;
 304
 305     if (done) return TRUE;
 306
 307     /* FIXME: this isn't 100% thread safe, as we won't catch accesses while initializing */
 308     if (hRunOnce == 0)
 309     {
 310         HANDLE hEvent = CreateEventW( NULL, TRUE, FALSE, NULL );
 311         if (InterlockedCompareExchangePointer( &hRunOnce, hEvent, 0 ) == 0)
 312         {
 313             BOOL ret;
 314             DWORD reserve;
 315
 316             /* ok, we're the winning thread */
 317             if (!(ret = VirtualProtect( DOSMEM_dosmem + DOSMEM_protect,
 318                                         DOSMEM_SIZE - DOSMEM_protect,
 319                                         PAGE_READWRITE, NULL )))
 320                 ERR("Cannot load access low 1Mb, DOS subsystem unavailable\n");
 321             RtlRemoveVectoredExceptionHandler( dosmem_handler );
 322
 323             /*
 324              * Reserve either:
 325              * - lowest 64k for NULL pointer catching (Win16)
 326              * - lowest 1k for interrupt handlers and
 327              *   another 0.5k for BIOS, DOS and intra-application
 328              *   areas (DOS)
 329              */
 330             if (DOSMEM_dosmem != DOSMEM_sysmem)
 331                 reserve = 0x10000; /* 64k */
 332             else
 333                 reserve = 0x600; /* 1.5k */
 334
 335             /*
 336              * Set DOS memory base and initialize conventional memory.
 337              */
 338             DOSMEM_FillBiosSegments();
 339             DOSMEM_FillIsrTable();
 340
 341             /* align root block to paragraph */
 342             DOSMEM_root_block = (MCB*)(DOSMEM_dosmem + reserve);
 343             DOSMEM_root_block->type = MCB_TYPE_LAST;
 344             DOSMEM_root_block->psp = MCB_PSP_FREE;
 345             DOSMEM_root_block->size = (DOSMEM_dosmem + 0x9fffc  - ((char*)DOSMEM_root_block)) >> 4;
 346
 347             TRACE("DOS conventional memory initialized, %d bytes free.\n",
 348                   DOSMEM_Available());
 349
 350             DOSVM_InitSegments();
 351
 352             SetEvent( hRunOnce );
 353             done = 1;
 354             return ret;
 355         }
 356         /* someone beat us here... */
 357         CloseHandle( hEvent );
 358     }
 359
 360     /* and wait for the winner to have finished */
 361     WaitForSingleObject( hRunOnce, INFINITE );
 362     return TRUE;
 363 }
 364
 365 /******************************************************************
 366  *              dosmem_handler
 367  *
 368  * Handler to catch access to our 1MB address space reserved for real memory
 369  */
 370 static LONG WINAPI dosmem_handler(EXCEPTION_POINTERS* except)
 371 {
 372     if (except->ExceptionRecord->ExceptionCode == EXCEPTION_ACCESS_VIOLATION)
 373     {
 374         char *addr = (char *)except->ExceptionRecord->ExceptionInformation[1];
 375         if (addr >= DOSMEM_dosmem + DOSMEM_protect && addr < DOSMEM_dosmem + DOSMEM_SIZE)
 376         {
 377             if (DOSMEM_InitDosMemory()) return EXCEPTION_CONTINUE_EXECUTION;
 378         }
 379     }
 380     return EXCEPTION_CONTINUE_SEARCH;
 381 }
 382
 383 /***********************************************************************
 384  *           DOSMEM_Init
 385  *
 386  * Create the dos memory segments, and store them into the KERNEL
 387  * exported values.
 388  */
 389 BOOL DOSMEM_Init(void)
 390 {
 391     void *addr = (void *)1;
 392     SIZE_T size = DOSMEM_SIZE - 1;
 393
 394     if (NtAllocateVirtualMemory( GetCurrentProcess(), &addr, 0, &size,
 395                                  MEM_RESERVE | MEM_COMMIT, PAGE_NOACCESS ))
 396     {
 397         ERR( "Cannot allocate DOS memory\n" );
 398         ExitProcess(1);
 399     }
 400
 401     if (addr <= (void *)DOSMEM_64KB)
 402     {
 403         DOSMEM_dosmem = 0;
 404         DOSMEM_protect = DOSMEM_64KB;
 405         DOSMEM_sysmem = (char *)0xf0000;  /* store sysmem in high addresses for now */
 406     }
 407     else
 408     {
 409         WARN( "First megabyte not available for DOS address space.\n" );
 410         DOSMEM_dosmem = addr;
 411         DOSMEM_protect = 0;
 412         DOSMEM_sysmem = DOSMEM_dosmem;
 413     }
 414
 415     RtlAddVectoredExceptionHandler(FALSE, dosmem_handler);
 416     DOSMEM_0000H = GLOBAL_CreateBlock( GMEM_FIXED, DOSMEM_sysmem,
 417                                        DOSMEM_64KB, 0, WINE_LDT_FLAGS_DATA );
 418     DOSMEM_BiosDataSeg = GLOBAL_CreateBlock( GMEM_FIXED, DOSMEM_sysmem + 0x400,
 419                                              0x100, 0, WINE_LDT_FLAGS_DATA );
 420     DOSMEM_BiosSysSeg = GLOBAL_CreateBlock( GMEM_FIXED, DOSMEM_dosmem + 0xf0000,
 421                                             DOSMEM_64KB, 0, WINE_LDT_FLAGS_DATA );
 422
 423     return TRUE;
 424 }
 425
 426 /***********************************************************************
 427  *           DOSMEM_MapLinearToDos
 428  *
 429  * Linear address to the DOS address space.
 430  */
 431 UINT DOSMEM_MapLinearToDos(LPVOID ptr)
 432 {
 433     if (((char*)ptr >= DOSMEM_dosmem) &&
 434         ((char*)ptr < DOSMEM_dosmem + DOSMEM_SIZE))
 435           return (char *)ptr - DOSMEM_dosmem;
 436     return (UINT)ptr;
 437 }
 438
 439
 440 /***********************************************************************
 441  *           DOSMEM_MapDosToLinear
 442  *
 443  * DOS linear address to the linear address space.
 444  */
 445 LPVOID DOSMEM_MapDosToLinear(UINT ptr)
 446 {
 447     if (ptr < DOSMEM_SIZE) return DOSMEM_dosmem + ptr;
 448     return (LPVOID)ptr;
 449 }
 450
 451
 452 /***********************************************************************
 453  *           DOSMEM_MapRealToLinear
 454  *
 455  * Real mode DOS address into a linear pointer
 456  */
 457 LPVOID DOSMEM_MapRealToLinear(DWORD x)
 458 {
 459    LPVOID       lin;
 460
 461    lin = DOSMEM_dosmem + HIWORD(x) * 16 + LOWORD(x);
 462    TRACE_(selector)("(0x%08x) returns %p.\n", x, lin );
 463    return lin;
 464 }
 465
 466 /***********************************************************************
 467  *           DOSMEM_AllocBlock
 468  *
 469  * Carve a chunk of the DOS memory block (without selector).
 470  */
 471 LPVOID DOSMEM_AllocBlock(UINT size, UINT16* pseg)
 472 {
 473     MCB *curr;
 474     MCB *next = NULL;
 475     WORD psp = DOSVM_psp;
 476
 477     DOSMEM_InitDosMemory();
 478
 479     curr = DOSMEM_root_block;
 480     if (!(psp = DOSVM_psp)) psp = MCB_PSP_DOS;
 481
 482     *pseg = 0;
 483
 484     TRACE( "(%04xh)\n", size );
 485
 486     /* round up to paragraph */
 487     size = (size + 15) >> 4;
 488
 489 #ifdef __DOSMEM_DEBUG__
 490     DOSMEM_Available();     /* checks the whole MCB list */
 491 #endif
 492
 493     /* loop over all MCB and search the next large enough MCB */
 494     while (curr)
 495     {
 496         if (!MCB_VALID (curr))
 497         {
 498             ERR( "MCB List Corrupt\n" );
 499             MCB_DUMP( curr );
 500             return NULL;
 501         }
 502         if (curr->psp == MCB_PSP_FREE)
 503         {
 504             DOSMEM_Collapse( curr );
 505             /* is it large enough (one paragraph for the MCB)? */
 506             if (curr->size >= size)
 507             {
 508                 if (curr->size > size)
 509                 {
 510                     /* split curr */
 511                     next = (MCB *) ((char*) curr + ((size+1) << 4));
 512                     next->psp = MCB_PSP_FREE;
 513                     next->size = curr->size - (size+1);
 514                     next->type = curr->type;
 515                     curr->type = MCB_TYPE_NORMAL;
 516                     curr->size = size;
 517                 }
 518                 /* curr is the found block */
 519                 curr->psp = psp;
 520                 if( pseg ) *pseg = (((char*)curr) + 16 - DOSMEM_dosmem) >> 4;
 521                 return (LPVOID) ((char*)curr + 16);
 522             }
 523         }
 524         curr = MCB_NEXT(curr);
 525     }
 526     return NULL;
 527 }
 528
 529 /***********************************************************************
 530  *           DOSMEM_FreeBlock
 531  */
 532 BOOL DOSMEM_FreeBlock(void* ptr)
 533 {
 534     MCB* mcb = (MCB*) ((char*)ptr - 16);
 535
 536     TRACE( "(%p)\n", ptr );
 537
 538 #ifdef __DOSMEM_DEBUG__
 539     DOSMEM_Available();
 540 #endif
 541
 542     if (!MCB_VALID (mcb))
 543     {
 544         ERR( "MCB invalid\n" );
 545         MCB_DUMP( mcb );
 546         return FALSE;
 547     }
 548
 549     mcb->psp = MCB_PSP_FREE;
 550     DOSMEM_Collapse( mcb );
 551     return TRUE;
 552 }
 553
 554 /***********************************************************************
 555  *           DOSMEM_ResizeBlock
 556  *
 557  * Resize DOS memory block in place. Returns block size or -1 on error.
 558  *
 559  * If exact is TRUE, returned value is either old or requested block
 560  * size. If exact is FALSE, block is expanded even if there is not
 561  * enough space for full requested block size.
 562  *
 563  * TODO: return also biggest block size
 564  */
 565 UINT DOSMEM_ResizeBlock(void *ptr, UINT size, BOOL exact)
 566 {
 567     MCB* mcb = (MCB*) ((char*)ptr - 16);
 568     MCB* next;
 569
 570     TRACE( "(%p,%04xh,%s)\n", ptr, size, exact ? "TRUE" : "FALSE" );
 571
 572     /* round up to paragraph */
 573     size = (size + 15) >> 4;
 574
 575 #ifdef __DOSMEM_DEBUG__
 576     DOSMEM_Available();
 577 #endif
 578
 579     if (!MCB_VALID (mcb))
 580     {
 581         ERR( "MCB invalid\n" );
 582         MCB_DUMP( mcb );
 583         return -1;
 584     }
 585
 586     /* resize needed? */
 587     if (mcb->size == size)
 588         return size << 4;
 589
 590     /* collapse free blocks */
 591     DOSMEM_Collapse( mcb );
 592
 593     /* shrink mcb ? */
 594     if (mcb->size > size)
 595     {
 596         next = (MCB *) ((char*)mcb + ((size+1) << 4));
 597         next->type = mcb->type;
 598         next->psp = MCB_PSP_FREE;
 599         next->size = mcb->size - (size+1);
 600         mcb->type = MCB_TYPE_NORMAL;
 601         mcb->size = size;
 602         return size << 4;
 603     }
 604
 605     if (!exact)
 606     {
 607         return mcb->size << 4;
 608     }
 609
 610     return -1;
 611 }
 612
 613 /***********************************************************************
 614  *           DOSMEM_Available
 615  */
 616 UINT DOSMEM_Available(void)
 617 {
 618     UINT  available = 0;
 619     UINT  total = 0;
 620     MCB *curr = DOSMEM_root_block;
 621     /* loop over all MCB and search the largest free MCB */
 622     while (curr)
 623     {
 624 #ifdef __DOSMEM_DEBUG__
 625         MCB_DUMP( curr );
 626 #endif
 627         if (!MCB_VALID (curr))
 628         {
 629             ERR( "MCB List Corrupt\n" );
 630             MCB_DUMP( curr );
 631             return 0;
 632         }
 633         if (curr->psp == MCB_PSP_FREE &&
 634             curr->size > available )
 635             available = curr->size;
 636
 637         total += curr->size + 1;
 638         curr = MCB_NEXT( curr );
 639     }
 640     TRACE( " %04xh of %04xh paragraphs available\n", available, total );
 641     return available << 4;
 642 }
 643
 644 /******************************************************************
 645  *              DOSMEM_MapDosLayout
 646  *
 647  * Initialize the first MB of memory to look like a real DOS setup
 648  */
 649 BOOL DOSMEM_MapDosLayout(void)
 650 {
 651     static int already_mapped;
 652
 653     if (!already_mapped)
 654     {
 655         if (DOSMEM_dosmem || !VirtualProtect( NULL, DOSMEM_SIZE, PAGE_EXECUTE_READWRITE, NULL ))
 656         {
 657             ERR( "Need full access to the first megabyte for DOS mode\n" );
 658             ExitProcess(1);
 659         }
 660         /* copy the BIOS and ISR area down */
 661         memcpy( DOSMEM_dosmem, DOSMEM_sysmem, 0x400 + 0x100 );
 662         DOSMEM_sysmem = DOSMEM_dosmem;
 663         SetSelectorBase( DOSMEM_0000H, 0 );
 664         SetSelectorBase( DOSMEM_BiosDataSeg, 0x400 );
 665         /* we may now need the actual interrupt stubs, and since we've just moved the
 666          * interrupt vector table away, we can fill the area with stubs instead... */
 667         DOSMEM_MakeIsrStubs();
 668         already_mapped = 1;
 669     }
 670     return TRUE;
 671 }