Looking into it, I found little reason to install the bioscall page in locore.
How about this change? (tested with PMAP_NEW only)

best regards
Matthias

Index: locore.s
===================================================================
RCS file: /cvsroot/src/sys/arch/i386/i386/locore.s,v
retrieving revision 1.188
diff -c -2 -r1.188 locore.s
*** locore.s	1998/02/22 08:19:05	1.188
--- locore.s	1998/03/03 18:54:38
***************
*** 51,55 ****
  #include "assym.h"
  #include "apm.h"
- #include "bioscall.h"
  
  #include <sys/errno.h>
--- 51,54 ----
***************
*** 475,484 ****
  #define	PROC0STACK	((1)              * NBPG)
  #define	SYSMAP		((1+UPAGES)       * NBPG)
! #if NBIOSCALL > 0
! #define	BIOSCALL_PDE_SPACE	1
! #else
! #define	BIOSCALL_PDE_SPACE	0
! #endif
! #define	TABLESIZE	((1+UPAGES+BIOSCALL_PDE_SPACE) * NBPG) /* + nkpde * NBPG */
  
  	/* Find end of kernel image. */
--- 474,478 ----
  #define	PROC0STACK	((1)              * NBPG)
  #define	SYSMAP		((1+UPAGES)       * NBPG)
! #define	TABLESIZE	((1+UPAGES) * NBPG) /* + nkpde * NBPG */
  
  	/* Find end of kernel image. */
***************
*** 594,600 ****
   * Construct a page table directory.
   */
! 	/* Install a PDE for temporary double map of kernel text. */
  	leal	(SYSMAP+PG_V|PG_KW)(%esi),%eax		# pte for KPT in proc 0,
! 	movl	%eax,(PROC0PDIR+0*4)(%esi)		# which is where temp maps!
  	/* Map kernel PDEs. */
  	movl	RELOC(_nkpde),%ecx			# for this many pde s,
--- 588,597 ----
   * Construct a page table directory.
   */
! 	/* Install a PDE for temporary double map of kernel. */
! 	movl	RELOC(_nkpde),%ecx			# for this many pde s,
! 	leal	(PROC0PDIR+0*4)(%esi),%ebx		# which is where temp maps!
  	leal	(SYSMAP+PG_V|PG_KW)(%esi),%eax		# pte for KPT in proc 0,
! 	fillkpt
! 
  	/* Map kernel PDEs. */
  	movl	RELOC(_nkpde),%ecx			# for this many pde s,
***************
*** 604,619 ****
  	leal	(PROC0PDIR+KPTDI*4)(%esi),%ebx		# offset of pde for kernel
  #endif
  	fillkpt
  
- #if NBIOSCALL > 0
- 	/* set up special identity mapping page table to get to
- 	   page 1 for BIOS trampoline.  store it in PTD 1 to start, though.... */
- 	
- 	/* use remaining page table page at %eax */
- 	movl	$1,%ecx				# only add mapping for one PTDE
- 	leal	(PROC0PDIR+1*4)(%esi),%ebx	# PTD entry address
- 	fillkpt
- #endif
- 	
  	/* Install a PDE recursively mapping page directory as a page table! */
  	leal	(PROC0PDIR+PG_V|PG_KW)(%esi),%eax	# pte for ptd
--- 601,607 ----
  	leal	(PROC0PDIR+KPTDI*4)(%esi),%ebx		# offset of pde for kernel
  #endif
+ 	leal	(SYSMAP+PG_V|PG_KW)(%esi),%eax		# pte for KPT in proc 0,
  	fillkpt
  
  	/* Install a PDE recursively mapping page directory as a page table! */
  	leal	(PROC0PDIR+PG_V|PG_KW)(%esi),%eax	# pte for ptd
***************
*** 639,651 ****
  
  begin:
! 	/* Now running relocated at KERNBASE.  Remove double mapping. */
! #if NBIOSCALL > 0
! 	/* move page table pointer for bios trampoline to final resting place */
! 	movl	(PROC0PDIR+1*4)(%esi),%edx
! 	movl	%edx,(PROC0PDIR+0*4)(%esi)
! 	movl	$0,(PROC0PDIR+1*4)(%esi)
! #else
! 	movl	$0,(PROC0PDIR+0*4)(%esi)
! #endif
  
  	/* Relocate atdevbase. */
--- 627,631 ----
  
  begin:
! 	/* Now running relocated at KERNBASE.  Double mapping is removed in
init386(). */
  
  	/* Relocate atdevbase. */
Index: machdep.c
===================================================================
RCS file: /cvsroot/src/sys/arch/i386/i386/machdep.c,v
retrieving revision 1.293
diff -c -2 -r1.293 machdep.c
*** machdep.c	1998/02/19 04:18:32	1.293
--- machdep.c	1998/03/03 18:54:42
***************
*** 1703,1706 ****
--- 1708,1719 ----
  	extern void consinit __P((void));
  
+ 	/* remove double mapping */
+ 	for (x = 0; x < nkpde; x++)
+ #if defined(PMAP_NEW)
+ 		*(PDP_BASE + x) = 0;
+ #else
+ 		PTD[x] = 0;
+ #endif
+ 
  	proc0.p_addr = proc0paddr;
  #if defined(PMAP_NEW)
***************
*** 1765,1769 ****
  
  #if NBIOSCALL > 0
! 	avail_start = 2*NBPG;	/* save us a page! */
  #else
  	avail_start = NBPG;	/* BIOS leaves data in low memory */
--- 1778,1783 ----
  
  #if NBIOSCALL > 0
! 	avail_start = 3*NBPG;	/* save us a page for trampoline code and
! 				 one additional PT page! */
  #else
  	avail_start = NBPG;	/* BIOS leaves data in low memory */
***************
*** 1786,1789 ****
--- 1800,1807 ----
  #endif
  
+ #if NBIOSCALL > 0
+ 	/* install page 2 (reserved above) as PT page for first 4M */
+ 	pmap_enter(pmap_kernel(), (u_long)vtopte(0), 2*NBPG, VM_PROT_ALL, TRUE);
+ #endif
  
  	pmap_enter(pmap_kernel(), idt_vaddr, idt_paddr, VM_PROT_ALL, TRUE);