memdisk/memdisk16.asm

   1 ;; -*- fundamental -*-
   2 ;; -----------------------------------------------------------------------
   3 ;;
   4 ;;   Copyright 1994-2008 H. Peter Anvin - All Rights Reserved
   5 ;;   Copyright 2009 Intel Corporation; author: H. Peter Anvin
   6 ;;
   7 ;;   This program is free software; you can redistribute it and/or modify
   8 ;;   it under the terms of the GNU General Public License as published by
   9 ;;   the Free Software Foundation, Inc., 53 Temple Place Ste 330,
  10 ;;   Boston MA 02111-1307, USA; either version 2 of the License, or
  11 ;;   (at your option) any later version; incorporated herein by reference.
  12 ;;
  13 ;; -----------------------------------------------------------------------
  14
  15 ;;
  16 ;; init16.asm
  17 ;;
  18 ;; Routine to initialize and to trampoline into 32-bit
  19 ;; protected memory.  This code is derived from bcopy32.inc and
  20 ;; com32.inc in the main SYSLINUX distribution.
  21 ;;
  22
  23 %include '../version.gen'
  24
  25 MY_CS           equ 0x0800              ; Segment address to use
  26 CS_BASE         equ (MY_CS << 4)        ; Corresponding address
  27
  28 ; Low memory bounce buffer
  29 BOUNCE_SEG      equ (MY_CS+0x1000)
  30
  31 %define DO_WBINVD 0
  32
  33                 section .rodata align=16
  34                 section .data   align=16
  35                 section .bss    align=16
  36                 section .stack  align=16 nobits
  37 stack           resb 512
  38 stack_end       equ $
  39
  40 ;; -----------------------------------------------------------------------
  41 ;;  Kernel image header
  42 ;; -----------------------------------------------------------------------
  43
  44                 section .text           ; Must be first in image
  45                 bits 16
  46
  47 cmdline         times 497 db 0          ; We put the command line here
  48 setup_sects     db 0
  49 root_flags      dw 0
  50 syssize         dw 0
  51 swap_dev        dw 0
  52 ram_size        dw 0
  53 vid_mode        dw 0
  54 root_dev        dw 0
  55 boot_flag       dw 0xAA55
  56
  57 _start:         jmp short start
  58
  59                 db "HdrS"               ; Header signature
  60                 dw 0x0203               ; Header version number
  61
  62 realmode_swtch  dw 0, 0                 ; default_switch, SETUPSEG
  63 start_sys_seg   dw 0x1000               ; obsolete
  64 version_ptr     dw memdisk_version-0x200        ; version string ptr
  65 type_of_loader  db 0                    ; Filled in by boot loader
  66 loadflags       db 1                    ; Please load high
  67 setup_move_size dw 0                    ; Unused
  68 code32_start    dd 0x100000             ; 32-bit start address
  69 ramdisk_image   dd 0                    ; Loaded ramdisk image address
  70 ramdisk_size    dd 0                    ; Size of loaded ramdisk
  71 bootsect_kludge dw 0, 0
  72 heap_end_ptr    dw 0
  73 pad1            dw 0
  74 cmd_line_ptr    dd 0                    ; Command line
  75 ramdisk_max     dd 0xffffffff           ; Highest allowed ramdisk address
  76
  77 ;
  78 ; These fields aren't real setup fields, they're poked in by the
  79 ; 32-bit code.
  80 ;
  81 b_esdi          dd 0                    ; ES:DI for boot sector invocation
  82 b_edx           dd 0                    ; EDX for boot sector invocation
  83 b_sssp          dd 0                    ; SS:SP on boot sector invocation
  84 b_csip          dd 0                    ; CS:IP on boot sector invocation
  85
  86                 section .rodata
  87 memdisk_version:
  88                 db "MEMDISK ", VERSION_STR, " ", DATE, 0
  89
  90 ;; -----------------------------------------------------------------------
  91 ;;  End kernel image header
  92 ;; -----------------------------------------------------------------------
  93
  94 ;
  95 ; Move ourselves down into memory to reduce the risk of conflicts;
  96 ; then canonicalize CS to match the other segments.
  97 ;
  98                 section .text
  99                 bits 16
 100 start:
 101                 mov ax,MY_CS
 102                 mov es,ax
 103                 movzx cx,byte [setup_sects]
 104                 inc cx                  ; Add one for the boot sector
 105                 shl cx,7                ; Convert to dwords
 106                 xor si,si
 107                 xor di,di
 108                 mov fs,si               ; fs <- 0
 109                 cld
 110                 rep movsd
 111                 mov ds,ax
 112                 mov ss,ax
 113                 mov esp,stack_end
 114                 jmp MY_CS:.next
 115 .next:
 116
 117 ;
 118 ; Copy the command line, if there is one
 119 ;
 120 copy_cmdline:
 121                 xor di,di               ; Bottom of our own segment (= "boot sector")
 122                 mov eax,[cmd_line_ptr]
 123                 and eax,eax
 124                 jz .endcmd              ; No command line
 125                 mov si,ax
 126                 shr eax,4               ; Convert to segment
 127                 and si,0x000F           ; Starting offset only
 128                 mov gs,ax
 129                 mov cx,496              ; Max number of bytes
 130 .copycmd:
 131                 gs lodsb
 132                 and al,al
 133                 jz .endcmd
 134                 stosb
 135                 loop .copycmd
 136 .endcmd:
 137                 xor al,al
 138                 stosb
 139
 140 ;
 141 ; Now jump to 32-bit code
 142 ;
 143                 sti
 144                 call init32
 145 ;
 146 ; When init32 returns, we have been set up, the new boot sector loaded,
 147 ; and we should go and and run the newly loaded boot sector.
 148 ;
 149 ; The setup function will have poked values into the setup area.
 150 ;
 151                 movzx edi,word [cs:b_esdi]
 152                 mov es,word [cs:b_esdi+2]
 153                 mov edx,[cs:b_edx]
 154
 155                 cli
 156                 xor esi,esi             ; No partition table involved
 157                 mov ds,si               ; Make all the segments consistent
 158                 mov fs,si
 159                 mov gs,si
 160                 lss sp,[cs:b_sssp]
 161                 movzx esp,sp
 162                 jmp far [cs:b_csip]
 163
 164 ;
 165 ; We enter protected mode, set up a flat 32-bit environment, run rep movsd
 166 ; and then exit.  IMPORTANT: This code assumes cs == MY_CS.
 167 ;
 168 ; This code is probably excessively anal-retentive in its handling of
 169 ; segments, but this stuff is painful enough as it is without having to rely
 170 ; on everything happening "as it ought to."
 171 ;
 172 DummyTSS        equ  0x580              ; Hopefully safe place in low mmoery
 173
 174                 section .data
 175
 176         ; desc base, limit, flags
 177 %macro  desc 3
 178         dd (%2 & 0xffff) | ((%1 & 0xffff) << 16)
 179         dd (%1 & 0xff000000) | (%2 & 0xf0000) | ((%3 & 0xf0ff) << 8) | ((%1 & 0x00ff0000) >> 16)
 180 %endmacro
 181
 182                 align 8, db 0
 183 call32_gdt:     dw call32_gdt_size-1    ; Null descriptor - contains GDT
 184 .adj1:          dd call32_gdt+CS_BASE   ; pointer for LGDT instruction
 185                 dw 0
 186
 187                 ; 0008: Dummy TSS to make Intel VT happy
 188                 ; Should never be actually accessed...
 189                 desc DummyTSS, 103, 0x8089
 190
 191                 ; 0010: Code segment, use16, readable, dpl 0, base CS_BASE, 64K
 192                 desc CS_BASE, 0xffff, 0x009b
 193
 194                 ; 0018: Data segment, use16, read/write, dpl 0, base CS_BASE, 64K
 195                 desc CS_BASE, 0xffff, 0x0093
 196
 197                 ; 0020: Code segment, use32, read/write, dpl 0, base 0, 4G
 198                 desc 0, 0xfffff, 0xc09b
 199
 200                 ; 0028: Data segment, use32, read/write, dpl 0, base 0, 4G
 201                 desc 0, 0xfffff, 0xc093
 202
 203 call32_gdt_size:        equ $-call32_gdt
 204
 205 err_a20:        db 'ERROR: A20 gate not responding!',13,10,0
 206
 207                 section .bss
 208                 alignb 4
 209 Return          resd 1                  ; Return value
 210 SavedSP         resw 1                  ; Place to save SP
 211 A20Tries        resb 1
 212
 213                 section .data
 214                 align 4, db 0
 215 Target          dd 0                    ; Target address
 216 Target_Seg      dw 20h                  ; Target CS
 217
 218 A20Type         dw 0                    ; Default = unknown
 219
 220                 section .text
 221                 bits 16
 222 ;
 223 ; Routines to enable and disable (yuck) A20.  These routines are gathered
 224 ; from tips from a couple of sources, including the Linux kernel and
 225 ; http://www.x86.org/.  The need for the delay to be as large as given here
 226 ; is indicated by Donnie Barnes of RedHat, the problematic system being an
 227 ; IBM ThinkPad 760EL.
 228 ;
 229 ; We typically toggle A20 twice for every 64K transferred.
 230 ;
 231 %define io_delay        call _io_delay
 232 %define IO_DELAY_PORT   80h             ; Invalid port (we hope!)
 233 %define disable_wait    32              ; How long to wait for a disable
 234
 235 %define A20_DUNNO       0               ; A20 type unknown
 236 %define A20_NONE        1               ; A20 always on?
 237 %define A20_BIOS        2               ; A20 BIOS enable
 238 %define A20_KBC         3               ; A20 through KBC
 239 %define A20_FAST        4               ; A20 through port 92h
 240
 241                 align 2, db 0
 242 A20List         dw a20_dunno, a20_none, a20_bios, a20_kbc, a20_fast
 243 A20DList        dw a20d_dunno, a20d_none, a20d_bios, a20d_kbc, a20d_fast
 244 a20_adjust_cnt  equ ($-A20List)/2
 245
 246 slow_out:       out dx, al              ; Fall through
 247
 248 _io_delay:      out IO_DELAY_PORT,al
 249                 out IO_DELAY_PORT,al
 250                 ret
 251
 252 enable_a20:
 253                 pushad
 254                 mov byte [A20Tries],255 ; Times to try to make this work
 255
 256 try_enable_a20:
 257
 258 ;
 259 ; Flush the caches
 260 ;
 261 %if DO_WBINVD
 262                 call try_wbinvd
 263 %endif
 264
 265 ;
 266 ; If the A20 type is known, jump straight to type
 267 ;
 268                 mov bp,[A20Type]
 269                 add bp,bp                       ; Convert to word offset
 270 .adj4:          jmp word [bp+A20List]
 271
 272 ;
 273 ; First, see if we are on a system with no A20 gate
 274 ;
 275 a20_dunno:
 276 a20_none:
 277                 mov byte [A20Type], A20_NONE
 278                 call a20_test
 279                 jnz a20_done
 280
 281 ;
 282 ; Next, try the BIOS (INT 15h AX=2401h)
 283 ;
 284 a20_bios:
 285                 mov byte [A20Type], A20_BIOS
 286                 mov ax,2401h
 287                 pushf                           ; Some BIOSes muck with IF
 288                 int 15h
 289                 popf
 290
 291                 call a20_test
 292                 jnz a20_done
 293
 294 ;
 295 ; Enable the keyboard controller A20 gate
 296 ;
 297 a20_kbc:
 298                 mov dl, 1                       ; Allow early exit
 299                 call empty_8042
 300                 jnz a20_done                    ; A20 live, no need to use KBC
 301
 302                 mov byte [A20Type], A20_KBC     ; Starting KBC command sequence
 303
 304                 mov al,0D1h                     ; Write output port
 305                 out 064h, al
 306                 call empty_8042_uncond
 307
 308                 mov al,0DFh                     ; A20 on
 309                 out 060h, al
 310                 call empty_8042_uncond
 311
 312                 ; Apparently the UHCI spec assumes that A20 toggle
 313                 ; ends with a null command (assumed to be for sychronization?)
 314                 ; Put it here to see if it helps anything...
 315                 mov al,0FFh                     ; Null command
 316                 out 064h, al
 317                 call empty_8042_uncond
 318
 319                 ; Verify that A20 actually is enabled.  Do that by
 320                 ; observing a word in low memory and the same word in
 321                 ; the HMA until they are no longer coherent.  Note that
 322                 ; we don't do the same check in the disable case, because
 323                 ; we don't want to *require* A20 masking (SYSLINUX should
 324                 ; work fine without it, if the BIOS does.)
 325 .kbc_wait:      push cx
 326                 xor cx,cx
 327 .kbc_wait_loop:
 328                 call a20_test
 329                 jnz a20_done_pop
 330                 loop .kbc_wait_loop
 331
 332                 pop cx
 333 ;
 334 ; Running out of options here.  Final attempt: enable the "fast A20 gate"
 335 ;
 336 a20_fast:
 337                 mov byte [A20Type], A20_FAST    ; Haven't used the KBC yet
 338                 in al, 092h
 339                 or al,02h
 340                 and al,~01h                     ; Don't accidentally reset the machine!
 341                 out 092h, al
 342
 343 .fast_wait:     push cx
 344                 xor cx,cx
 345 .fast_wait_loop:
 346                 call a20_test
 347                 jnz a20_done_pop
 348                 loop .fast_wait_loop
 349
 350                 pop cx
 351
 352 ;
 353 ; Oh bugger.  A20 is not responding.  Try frobbing it again; eventually give up
 354 ; and report failure to the user.
 355 ;
 356
 357                 dec byte [A20Tries]
 358                 jnz try_enable_a20
 359
 360
 361                 ; Error message time
 362                 mov si,err_a20
 363 print_err:
 364                 lodsb
 365                 and al,al
 366                 jz die
 367                 mov bx,7
 368                 mov ah,0xe
 369                 int 10h
 370                 jmp print_err
 371
 372
 373 die:
 374                 sti
 375 .hlt:           hlt
 376                 jmp short .hlt
 377
 378 ;
 379 ; A20 unmasked, proceed...
 380 ;
 381 a20_done_pop:   pop cx
 382 a20_done:       popad
 383                 ret
 384
 385 ;
 386 ; This routine tests if A20 is enabled (ZF = 0).  This routine
 387 ; must not destroy any register contents.
 388 ;
 389
 390 ; This is the INT 1Fh vector, which is standard PCs is used by the
 391 ; BIOS when the screen is in graphics mode.  Even if it is, it points to
 392 ; data, not code, so it should be safe enough to fiddle with.
 393 A20Test         equ (1Fh*4)
 394
 395 a20_test:
 396                 push ds
 397                 push es
 398                 push cx
 399                 push eax
 400                 xor ax,ax
 401                 mov ds,ax               ; DS == 0
 402                 dec ax
 403                 mov es,ax               ; ES == 0FFFFh
 404                 mov cx,32               ; Loop count
 405                 mov eax,[A20Test]
 406                 cmp eax,[es:A20Test+10h]
 407                 jne .a20_done
 408                 push eax
 409 .a20_wait:
 410                 inc eax
 411                 mov [A20Test],eax
 412                 io_delay
 413                 cmp eax,[es:A20Test+10h]
 414                 loopz .a20_wait
 415                 pop dword [A20Test]     ; Restore original value
 416 .a20_done:
 417                 pop eax
 418                 pop cx
 419                 pop es
 420                 pop ds
 421                 ret
 422
 423 disable_a20:
 424                 pushad
 425 ;
 426 ; Flush the caches
 427 ;
 428 %if DO_WBINVD
 429                 call try_wbinvd
 430 %endif
 431
 432                 mov bp,[A20Type]
 433                 add bp,bp                       ; Convert to word offset
 434 .adj5:          jmp word [bp+A20DList]
 435
 436 a20d_bios:
 437                 mov ax,2400h
 438                 pushf                           ; Some BIOSes muck with IF
 439                 int 15h
 440                 popf
 441                 jmp short a20d_snooze
 442
 443 ;
 444 ; Disable the "fast A20 gate"
 445 ;
 446 a20d_fast:
 447                 in al, 092h
 448                 and al,~03h
 449                 out 092h, al
 450                 jmp short a20d_snooze
 451
 452 ;
 453 ; Disable the keyboard controller A20 gate
 454 ;
 455 a20d_kbc:
 456                 call empty_8042_uncond
 457
 458                 mov al,0D1h
 459                 out 064h, al            ; Write output port
 460                 call empty_8042_uncond
 461
 462                 mov al,0DDh             ; A20 off
 463                 out 060h, al
 464                 call empty_8042_uncond
 465
 466                 mov al,0FFh             ; Null command/synchronization
 467                 out 064h, al
 468                 call empty_8042_uncond
 469
 470                 ; Wait a bit for it to take effect
 471 a20d_snooze:
 472                 push cx
 473                 mov cx, disable_wait
 474 .delayloop:     call a20_test
 475                 jz .disabled
 476                 loop .delayloop
 477 .disabled:      pop cx
 478 a20d_dunno:
 479 a20d_none:
 480                 popad
 481                 ret
 482
 483 ;
 484 ; Routine to empty the 8042 KBC controller.  If dl != 0
 485 ; then we will test A20 in the loop and exit if A20 is
 486 ; suddenly enabled.
 487 ;
 488 empty_8042_uncond:
 489                 xor dl,dl
 490 empty_8042:
 491                 call a20_test
 492                 jz .a20_on
 493                 and dl,dl
 494                 jnz .done
 495 .a20_on:        io_delay
 496                 in al, 064h             ; Status port
 497                 test al,1
 498                 jz .no_output
 499                 io_delay
 500                 in al, 060h             ; Read input
 501                 jmp short empty_8042
 502 .no_output:
 503                 test al,2
 504                 jnz empty_8042
 505                 io_delay
 506 .done:          ret
 507
 508 ;
 509 ; Execute a WBINVD instruction if possible on this CPU
 510 ;
 511 %if DO_WBINVD
 512 try_wbinvd:
 513                 wbinvd
 514                 ret
 515 %endif
 516
 517                 section .bss
 518                 alignb 4
 519 PMESP           resd 1                  ; Protected mode %esp
 520
 521                 section .idt nobits align=4096
 522                 alignb 4096
 523 pm_idt          resb 4096               ; Protected-mode IDT, followed by interrupt stubs
 524
 525
 526
 527
 528 pm_entry:       equ 0x100000
 529
 530                 section .rodata
 531                 align 2, db 0
 532 call32_rmidt:
 533                 dw 0ffffh               ; Limit
 534                 dd 0                    ; Address
 535
 536                 section .data
 537                 alignb 2
 538 call32_pmidt:
 539                 dw 8*256                ; Limit
 540                 dd 0                    ; Address (entered later)
 541
 542                 section .text
 543 ;
 544 ; This is the main entrypoint in this function
 545 ;
 546 init32:
 547                 mov bx,call32_call_start        ; Where to go in PM
 548
 549 ;
 550 ; Enter protected mode.  BX contains the entry point relative to the
 551 ; real-mode CS.
 552 ;
 553 call32_enter_pm:
 554                 mov ax,cs
 555                 mov ds,ax
 556                 movzx ebp,ax
 557                 shl ebp,4               ; EBP <- CS_BASE
 558                 movzx ebx,bx
 559                 add ebx,ebp             ; entry point += CS_BASE
 560                 cli
 561                 mov [SavedSP],sp
 562                 cld
 563                 call enable_a20
 564                 mov byte [call32_gdt+8+5],89h   ; Mark TSS unbusy
 565                 o32 lgdt [call32_gdt]   ; Set up GDT
 566                 o32 lidt [call32_pmidt] ; Set up IDT
 567                 mov eax,cr0
 568                 or al,1
 569                 mov cr0,eax             ; Enter protected mode
 570                 jmp 20h:strict dword .in_pm+CS_BASE
 571 .pm_jmp         equ $-6
 572
 573
 574                 bits 32
 575 .in_pm:
 576                 xor eax,eax             ; Available for future use...
 577                 mov fs,eax
 578                 mov gs,eax
 579                 lldt ax
 580
 581                 mov al,28h              ; Set up data segments
 582                 mov es,eax
 583                 mov ds,eax
 584                 mov ss,eax
 585
 586                 mov al,08h
 587                 ltr ax
 588
 589                 mov esp,[ebp+PMESP]     ; Load protmode %esp if available
 590                 jmp ebx                 ; Go to where we need to go
 591
 592 ;
 593 ; This is invoked before first dispatch of the 32-bit code, in 32-bit mode
 594 ;
 595 call32_call_start:
 596                 ;
 597                 ; Set up a temporary stack in the bounce buffer;
 598                 ; start32.S will override this to point us to the real
 599                 ; high-memory stack.
 600                 ;
 601                 mov esp, (BOUNCE_SEG << 4) + 0x10000
 602
 603                 push dword call32_enter_rm.rm_jmp+CS_BASE
 604                 push dword call32_enter_pm.pm_jmp+CS_BASE
 605                 push dword stack_end            ; RM size
 606                 push dword call32_gdt+CS_BASE
 607                 push dword call32_handle_interrupt+CS_BASE
 608                 push dword CS_BASE              ; Segment base
 609                 push dword (BOUNCE_SEG << 4)    ; Bounce buffer address
 610                 push dword call32_syscall+CS_BASE ; Syscall entry point
 611
 612                 call pm_entry-CS_BASE           ; Run the program...
 613
 614                 ; ... fall through to call32_exit ...
 615
 616 call32_exit:
 617                 mov bx,call32_done      ; Return to command loop
 618
 619 call32_enter_rm:
 620                 ; Careful here... the PM code may have relocated the
 621                 ; entire RM code, so we need to figure out exactly
 622                 ; where we are executing from.  If the PM code has
 623                 ; relocated us, it *will* have adjusted the GDT to
 624                 ; match, though.
 625                 call .here
 626 .here:          pop ebp
 627                 sub ebp,.here
 628                 o32 sidt [ebp+call32_pmidt]
 629                 cli
 630                 cld
 631                 mov [ebp+PMESP],esp     ; Save exit %esp
 632                 xor esp,esp             ; Make sure the high bits are zero
 633                 jmp 10h:.in_pm16        ; Return to 16-bit mode first
 634
 635                 bits 16
 636 .in_pm16:
 637                 mov ax,18h              ; Real-mode-like segment
 638                 mov es,ax
 639                 mov ds,ax
 640                 mov ss,ax
 641                 mov fs,ax
 642                 mov gs,ax
 643
 644                 lidt [call32_rmidt]     ; Real-mode IDT (rm needs no GDT)
 645                 mov eax,cr0
 646                 and al,~1
 647                 mov cr0,eax
 648                 jmp MY_CS:.in_rm
 649 .rm_jmp         equ $-2
 650
 651 .in_rm:                                 ; Back in real mode
 652                 mov ax,cs
 653                 mov ds,ax
 654                 mov es,ax
 655                 mov fs,ax
 656                 mov gs,ax
 657                 mov ss,ax
 658                 mov sp,[SavedSP]        ; Restore stack
 659                 jmp bx                  ; Go to whereever we need to go...
 660
 661 call32_done:
 662                 call disable_a20
 663                 sti
 664                 ret
 665
 666 ;
 667 ; 16-bit support code
 668 ;
 669                 bits 16
 670
 671 ;
 672 ; 16-bit interrupt-handling code
 673 ;
 674 call32_int_rm:
 675                 pushf                           ; Flags on stack
 676                 push cs                         ; Return segment
 677                 push word .cont                 ; Return address
 678                 push dword edx                  ; Segment:offset of IVT entry
 679                 retf                            ; Invoke IVT routine
 680 .cont:          ; ... on resume ...
 681                 mov bx,call32_int_resume
 682                 jmp call32_enter_pm             ; Go back to PM
 683
 684 ;
 685 ; 16-bit system call handling code
 686 ;
 687 call32_sys_rm:
 688                 pop gs
 689                 pop fs
 690                 pop es
 691                 pop ds
 692                 popad
 693                 popfd
 694                 retf                            ; Invoke routine
 695 .return:
 696                 pushfd
 697                 pushad
 698                 push ds
 699                 push es
 700                 push fs
 701                 push gs
 702                 mov bx,call32_sys_resume
 703                 jmp call32_enter_pm
 704
 705 ;
 706 ; 32-bit support code
 707 ;
 708                 bits 32
 709
 710 ;
 711 ; This is invoked on getting an interrupt in protected mode.  At
 712 ; this point, we need to context-switch to real mode and invoke
 713 ; the interrupt routine.
 714 ;
 715 ; When this gets invoked, the registers are saved on the stack and
 716 ; AL contains the register number.
 717 ;
 718 call32_handle_interrupt:
 719                 movzx eax,al
 720                 xor ebx,ebx             ; Actually makes the code smaller
 721                 mov edx,[ebx+eax*4]     ; Get the segment:offset of the routine
 722                 mov bx,call32_int_rm
 723                 jmp call32_enter_rm     ; Go to real mode
 724
 725 call32_int_resume:
 726                 popad
 727                 iret
 728
 729 ;
 730 ; Syscall invocation.  We manifest a structure on the real-mode stack,
 731 ; containing the call32sys_t structure from <call32.h> as well as
 732 ; the following entries (from low to high address):
 733 ; - Target offset
 734 ; - Target segment
 735 ; - Return offset
 736 ; - Return segment (== real mode cs)
 737 ; - Return flags
 738 ;
 739 call32_syscall:
 740                 pushfd                  ; Save IF among other things...
 741                 pushad                  ; We only need to save some, but...
 742                 cld
 743                 call .here
 744 .here:          pop ebp
 745                 sub ebp,.here
 746
 747                 movzx edi,word [ebp+SavedSP]
 748                 sub edi,54              ; Allocate 54 bytes
 749                 mov [ebp+SavedSP],di
 750                 add edi,ebp             ; Create linear address
 751
 752                 mov esi,[esp+11*4]      ; Source regs
 753                 xor ecx,ecx
 754                 mov cl,11               ; 44 bytes to copy
 755                 rep movsd
 756
 757                 movzx eax,byte [esp+10*4] ; Interrupt number
 758                 ; ecx == 0 here; adding it to the EA makes the
 759                 ; encoding smaller
 760                 mov eax,[ecx+eax*4]     ; Get IVT entry
 761                 stosd                   ; Save in stack frame
 762                 mov ax,call32_sys_rm.return     ; Return offset
 763                 stosw                           ; Save in stack frame
 764                 mov eax,ebp
 765                 shr eax,4                       ; Return segment
 766                 stosw                           ; Save in stack frame
 767                 mov eax,[edi-12]        ; Return flags
 768                 and eax,0x200cd7        ; Mask (potentially) unsafe flags
 769                 mov [edi-12],eax        ; Primary flags entry
 770                 stosw                   ; Return flags
 771
 772                 mov bx,call32_sys_rm
 773                 jmp call32_enter_rm     ; Go to real mode
 774
 775                 ; On return, the 44-byte return structure is on the
 776                 ; real-mode stack.  call32_enter_pm will leave ebp
 777                 ; pointing to the real-mode base.
 778 call32_sys_resume:
 779                 movzx esi,word [ebp+SavedSP]
 780                 mov edi,[esp+12*4]      ; Dest regs
 781                 add esi,ebp             ; Create linear address
 782                 and edi,edi             ; NULL pointer?
 783                 jnz .do_copy
 784 .no_copy:       mov edi,esi             ; Do a dummy copy-to-self
 785 .do_copy:       xor ecx,ecx
 786                 mov cl,11               ; 44 bytes
 787                 rep movsd               ; Copy register block
 788
 789                 add word [ebp+SavedSP],44       ; Remove from stack
 790
 791                 popad
 792                 popfd
 793                 ret                     ; Return to 32-bit program