2 * librm: a library for interfacing to real-mode code
4 * Michael Brown <mbrown@fensystems.co.uk>
8 FILE_LICENCE ( GPL2_OR_LATER )
10 /* Drag in local definitions */
13 /* For switches to/from protected mode */
16 /* Size of various C data structures */
17 #define SIZEOF_I386_SEG_REGS 12
18 #define SIZEOF_I386_REGS 32
19 #define SIZEOF_REAL_MODE_REGS ( SIZEOF_I386_SEG_REGS + SIZEOF_I386_REGS )
20 #define SIZEOF_I386_FLAGS 4
21 #define SIZEOF_I386_ALL_REGS ( SIZEOF_REAL_MODE_REGS + SIZEOF_I386_FLAGS )
25 /****************************************************************************
26 * Global descriptor table
28 * Call init_librm to set up the GDT before attempting to use any
29 * protected-mode code.
31 * NOTE: This must be located before prot_to_real, otherwise gas
32 * throws a "can't handle non absolute segment in `ljmp'" error due to
33 * not knowing the value of REAL_CS when the ljmp is encountered.
35 * Note also that putting ".word gdt_end - gdt - 1" directly into
36 * gdt_limit, rather than going via gdt_length, will also produce the
37 * "non absolute segment" error. This is most probably a bug in gas.
38 ****************************************************************************
40 .section ".data16", "aw", @progbits
43 gdtr: /* The first GDT entry is unused, the GDTR can fit here. */
44 gdt_limit: .word gdt_length - 1
48 .org gdt + VIRTUAL_CS, 0
49 virtual_cs: /* 32 bit protected mode code segment, virtual addresses */
51 .byte 0, 0x9f, 0xcf, 0
53 .org gdt + VIRTUAL_DS, 0
54 virtual_ds: /* 32 bit protected mode data segment, virtual addresses */
56 .byte 0, 0x93, 0xcf, 0
58 .org gdt + PHYSICAL_CS, 0
59 physical_cs: /* 32 bit protected mode code segment, physical addresses */
61 .byte 0, 0x9f, 0xcf, 0
63 .org gdt + PHYSICAL_DS, 0
64 physical_ds: /* 32 bit protected mode data segment, physical addresses */
66 .byte 0, 0x93, 0xcf, 0
69 real_cs: /* 16 bit flat real mode code segment */
71 .byte 0, 0x9b, 0x8f, 0
74 real_ds: /* 16 bit flat real mode data segment */
76 .byte 0, 0x93, 0x8f, 0
79 .equ gdt_length, gdt_end - gdt
81 /****************************************************************************
82 * init_librm (real-mode far call, 16-bit real-mode far return address)
84 * Initialise the GDT ready for transitions to protected mode.
87 * %cs : .text16 segment
88 * %ds : .data16 segment
89 * %edi : Physical base of protected-mode code (virt_offset)
90 ****************************************************************************
92 .section ".text16", "ax", @progbits
96 /* Preserve registers */
100 /* Store _virt_offset and set up virtual_cs and virtual_ds segments */
102 movw $virtual_cs, %bx
104 movw $virtual_ds, %bx
106 movl %edi, _virt_offset
108 /* Negate virt_offset */
111 /* Store rm_cs and _text16, set up real_cs segment */
118 addr32 leal (%eax, %edi), %ebx
121 /* Store rm_ds and _data16, set up real_ds segment */
128 addr32 leal (%eax, %edi), %ebx
131 /* Set GDT and IDT base */
136 /* Restore registers */
142 .section ".text16", "ax", @progbits
151 idt_init: /* Reuse the return opcode here */
154 /****************************************************************************
155 * real_to_prot (real-mode near call, 32-bit virtual return address)
157 * Switch from 16-bit real-mode to 32-bit protected mode with virtual
158 * addresses. The real-mode %ss:sp is stored in rm_ss and rm_sp, and
159 * the protected-mode %esp is restored from the saved pm_esp.
160 * Interrupts are disabled. All other registers may be destroyed.
162 * The return address for this function should be a 32-bit virtual
166 * %ecx : number of bytes to move from RM stack to PM stack
168 ****************************************************************************
170 .section ".text16", "ax", @progbits
173 /* Enable A20 line */
175 /* A failure at this point is fatal, and there's nothing we
176 * can do about it other than lock the machine to make the
177 * problem immediately visible.
181 /* Make sure we have our data segment available */
185 /* Add _virt_offset, _text16 and _data16 to stack to be
186 * copied, and also copy the return address.
191 addw $16, %cx /* %ecx must be less than 64kB anyway */
193 /* Real-mode %ss:%sp => %ebp:%edx and virtual address => %esi */
199 addr32 leal (%eax,%edx), %esi
200 subl _virt_offset, %esi
202 /* Switch to protected mode */
209 data32 ljmp $VIRTUAL_CS, $1f
210 .section ".text", "ax", @progbits
213 /* Set up protected-mode data segments and stack pointer */
214 movw $VIRTUAL_DS, %ax
222 /* Record real-mode %ss:sp (after removal of data) */
227 /* Move data from RM stack to PM stack */
232 /* Publish virt_offset, text16 and data16 for PM code to use */
237 /* Return to virtual address */
240 /* Default IDTR with no interrupts */
241 .section ".data16", "aw", @progbits
245 .word 0xffff /* limit */
248 /****************************************************************************
249 * prot_to_real (protected-mode near call, 32-bit real-mode return address)
251 * Switch from 32-bit protected mode with virtual addresses to 16-bit
252 * real mode. The protected-mode %esp is stored in pm_esp and the
253 * real-mode %ss:sp is restored from the saved rm_ss and rm_sp. The
254 * high word of the real-mode %esp is set to zero. All real-mode data
255 * segment registers are loaded from the saved rm_ds. Interrupts are
256 * *not* enabled, since we want to be able to use prot_to_real in an
257 * ISR. All other registers may be destroyed.
259 * The return address for this function should be a 32-bit (sic)
260 * real-mode offset within .code16.
263 * %ecx : number of bytes to move from PM stack to RM stack
265 ****************************************************************************
267 .section ".text", "ax", @progbits
270 /* Add return address to data to be moved to RM stack */
273 /* Real-mode %ss:sp => %ebp:edx and virtual address => %edi */
279 leal (%eax,%edx), %edi
280 subl virt_offset, %edi
282 /* Move data from PM stack to RM stack */
286 /* Record protected-mode %esp (after removal of data) */
289 /* Load real-mode segment limits */
297 .section ".text16", "ax", @progbits
300 /* Switch to real mode */
304 ljmp *p2r_jump_vector
307 /* Set up real-mode data segments and stack pointer */
316 /* Reset IDTR to the real-mode defaults */
319 /* Return to real-mode address */
323 /* Real-mode code and data segments. Assigned by the call to
324 * init_librm. rm_cs doubles as the segment part of the jump
325 * vector used by prot_to_real. rm_ds is located in .text16
326 * rather than .data16 because code needs to be able to locate
329 .section ".data16", "aw", @progbits
331 .word p2r_jump_target
335 .section ".text16.data", "aw", @progbits
338 /****************************************************************************
339 * prot_call (real-mode far call, 16-bit real-mode far return address)
341 * Call a specific C function in the protected-mode code. The
342 * prototype of the C function must be
343 * void function ( struct i386_all_regs *ix86 );
344 * ix86 will point to a struct containing the real-mode registers
345 * at entry to prot_call.
347 * All registers will be preserved across prot_call(), unless the C
348 * function explicitly overwrites values in ix86. Interrupt status
349 * and GDT will also be preserved. Gate A20 will be enabled.
351 * Note that prot_call() does not rely on the real-mode stack
352 * remaining intact in order to return, since everything relevant is
353 * copied to the protected-mode stack for the duration of the call.
354 * In particular, this means that a real-mode prefix can make a call
355 * to main() which will return correctly even if the prefix's stack
356 * gets vapourised during the Etherboot run. (The prefix cannot rely
357 * on anything else on the stack being preserved, so should move any
358 * critical data to registers before calling main()).
361 * function : virtual address of protected-mode function to call
364 * pushl $pxe_api_call
367 * to call in to the C function
368 * void pxe_api_call ( struct i386_all_regs *ix86 );
369 ****************************************************************************
372 #define PC_OFFSET_GDT ( 0 )
373 #define PC_OFFSET_IDT ( PC_OFFSET_GDT + 8 /* pad to 8 to keep alignment */ )
374 #define PC_OFFSET_IX86 ( PC_OFFSET_IDT + 8 /* pad to 8 to keep alignment */ )
375 #define PC_OFFSET_RETADDR ( PC_OFFSET_IX86 + SIZEOF_I386_ALL_REGS )
376 #define PC_OFFSET_FUNCTION ( PC_OFFSET_RETADDR + 4 )
377 #define PC_OFFSET_END ( PC_OFFSET_FUNCTION + 4 )
379 .section ".text16", "ax", @progbits
383 /* Preserve registers, flags and GDT on external RM stack */
397 /* For sanity's sake, clear the direction flag as soon as possible */
400 /* Switch to protected mode and move register dump to PM stack */
401 movl $PC_OFFSET_END, %ecx
404 .section ".text", "ax", @progbits
408 leal PC_OFFSET_IX86(%esp), %eax
410 call *(PC_OFFSET_FUNCTION+4)(%esp)
411 popl %eax /* discard */
413 /* Switch to real mode and move register dump back to RM stack */
414 movl $PC_OFFSET_END, %ecx
417 .section ".text16", "ax", @progbits
420 /* Reload GDT and IDT, restore registers and flags and return */
424 addw $20, %sp /* also skip %cs and %ss */
430 /* popal skips %esp. We therefore want to do "movl -20(%sp),
431 * %esp", but -20(%sp) is not a valid 80386 expression.
432 * Fortunately, prot_to_real() zeroes the high word of %esp, so
433 * we can just use -20(%esp) instead.
435 addr32 movl -20(%esp), %esp
439 /****************************************************************************
440 * real_call (protected-mode near call, 32-bit virtual return address)
442 * Call a real-mode function from protected-mode code.
444 * The non-segment register values will be passed directly to the
445 * real-mode code. The segment registers will be set as per
446 * prot_to_real. The non-segment register values set by the real-mode
447 * function will be passed back to the protected-mode caller. A
448 * result of this is that this routine cannot be called directly from
449 * C code, since it clobbers registers that the C ABI expects the
450 * callee to preserve.
452 * librm.h defines a convenient macro REAL_CODE() for using real_call.
453 * See librm.h and realmode.h for details and examples.
456 * (32-bit) near pointer to real-mode function to call
459 ****************************************************************************
462 #define RC_OFFSET_PRESERVE_REGS ( 0 )
463 #define RC_OFFSET_RETADDR ( RC_OFFSET_PRESERVE_REGS + SIZEOF_I386_REGS )
464 #define RC_OFFSET_FUNCTION ( RC_OFFSET_RETADDR + 4 )
465 #define RC_OFFSET_END ( RC_OFFSET_FUNCTION + 4 )
467 .section ".text", "ax", @progbits
471 /* Create register dump and function pointer copy on PM stack */
473 pushl RC_OFFSET_FUNCTION(%esp)
475 /* Switch to real mode and move register dump to RM stack */
476 movl $( RC_OFFSET_RETADDR + 4 /* function pointer copy */ ), %ecx
479 .section ".text16", "ax", @progbits
482 /* Call real-mode function */
488 /* For sanity's sake, clear the direction flag as soon as possible */
491 /* Switch to protected mode and move register dump back to PM stack */
492 movl $RC_OFFSET_RETADDR, %ecx
495 .section ".text", "ax", @progbits
498 /* Restore registers and return */
503 /* Function vector, used because "call xx(%sp)" is not a valid
506 .section ".data16", "aw", @progbits
507 rc_function: .word 0, 0
509 /****************************************************************************
510 * Stored real-mode and protected-mode stack pointers
512 * The real-mode stack pointer is stored here whenever real_to_prot
513 * is called and restored whenever prot_to_real is called. The
514 * converse happens for the protected-mode stack pointer.
516 * Despite initial appearances this scheme is, in fact re-entrant,
517 * because program flow dictates that we always return via the point
518 * we left by. For example:
522 * Print a text string
532 * At point 1, the RM mode stack value, say RPXE, is stored in
533 * rm_ss,sp. We want this value to still be present in rm_ss,sp when
536 * At point 2, the RM stack value is restored from RPXE. At point 3,
537 * the RM stack value is again stored in rm_ss,sp. This *does*
538 * overwrite the RPXE that we have stored there, but it's the same
539 * value, since the code between points 2 and 3 has managed to return
541 ****************************************************************************
543 .section ".data", "aw", @progbits
548 pm_esp: .long _estack
550 /****************************************************************************
551 * Virtual address offsets
553 * These are used by the protected-mode code to map between virtual
554 * and physical addresses, and to access variables in the .text16 or
556 ****************************************************************************
558 /* Internal copies, created by init_librm (which runs in real mode) */
559 .section ".data16", "aw", @progbits
560 _virt_offset: .long 0
564 /* Externally-visible copies, created by real_to_prot */
565 .section ".data", "aw", @progbits