diff --git a/plat/msdos386/boot.s b/plat/msdos386/boot.s
index 618e17163..282dcb866 100644
--- a/plat/msdos386/boot.s
+++ b/plat/msdos386/boot.s
@@ -18,15 +18,24 @@ begtext:
     ! On entry, the stub has cs and ds pointing at the 32-bit
     ! segment, but ss is still pointing at the 16-bit segment.
     !
-    !   si:di memory handle of linear block
     !   ax: pmode code segment of stub
-    !   dx: pointer to realloc routine
-    !   fs: data segment (just a clone of the code segment)
+    !   bx: pmode data segment of stub
+    !   cx: rmode segment of stub
+    !   dx: pointer to realloc routine (in stub)e
+    !   si: pointer to interrupt routine (in stub)
+    !   di: pointer to transfer buffer (in stub)
 
     ! Resize the segment to include the BSS.
 
     o16 cseg mov (realloc_ptr+4), ax
     cseg mov (realloc_ptr+0), edx
+    cseg o16 mov (pmode_cs), ax
+    cseg o16 mov (pmode_ds), bx
+    cseg o16 mov (rmode), cx
+    cseg o16 mov (interrupt_ptr+4), ax
+    cseg mov (interrupt_ptr+0), esi
+    cseg o16 mov (transfer_buffer_ptr), di
+
     mov eax, endbss
     cseg callf (realloc_ptr)
 
@@ -180,10 +189,19 @@ al_exit:
     movb ah, 0x4c
     int 0x21
 
-    ! This must be in the code segment due to bootstrap issues.
-realloc_ptr:
-    .data2 0
-    .data4 0
+    ! These must be in the code segment due to bootstrap issues.
+realloc_ptr:         .space 6 ! far
+interrupt_ptr:       .space 6 ! far
+transfer_buffer_ptr: .space 2 ! near
+rmode:               .space 2
+pmode_cs:            .space 2
+pmode_ds:            .space 2
+
+.define interrupt_ptr
+.define rmode
+.define pmode_cs
+.define pmode_ds
+.define transfer_buffer_ptr
 
 ! Define symbols at the beginning of our various segments, so that we can find
 ! them. (Except .text, which has already been done.)
@@ -210,7 +228,7 @@ no_room_msg: .ascii 'No room$'
 .comm _psp, 256
 
 .sect .bss
-    .space 512
+    .space 32*1024
 .stack:
 
 ! vim: ts=4 sw=4 et ft=asm
diff --git a/plat/msdos386/descr b/plat/msdos386/descr
index 718424e21..d7be3eb1e 100644
--- a/plat/msdos386/descr
+++ b/plat/msdos386/descr
@@ -39,7 +39,6 @@ name as
 	program {EM}/lib/ack/{PLATFORM}/as
 	args - -o > <
 	prep cond
-
 end
 name led
 	from .o.a
diff --git a/plat/msdos386/libsys/build.lua b/plat/msdos386/libsys/build.lua
index 70c7eaa17..4d349eb0f 100644
--- a/plat/msdos386/libsys/build.lua
+++ b/plat/msdos386/libsys/build.lua
@@ -16,7 +16,8 @@ acklibrary {
 		"./sys_rawcreat.s",
 		"./sys_rawlseek.s",
 		"./sys_rawopen.s",
-		"./sys_rawrw.s",
+		"./sys_rawread.s",
+		"./sys_rawwrite.s",
 		"./sys_xret.s",
 		"./unlink.s",
 		"plat/msdos/libsys+srcs",
diff --git a/plat/msdos386/libsys/close.s b/plat/msdos386/libsys/close.s
index b8c2f8a15..5e52a638b 100644
--- a/plat/msdos386/libsys/close.s
+++ b/plat/msdos386/libsys/close.s
@@ -16,8 +16,8 @@
 
 .define _close
 _close:
-	mov bx, sp
-	mov bx, 2(bx)
+	mov ebx, sp
+	mov ebx, 4(ebx)
 	movb ah, 0x3E
 	int 0x21
 	jmp .sys_zret
diff --git a/plat/msdos386/libsys/getpid.s b/plat/msdos386/libsys/getpid.s
index 5aa80638b..d2a250491 100644
--- a/plat/msdos386/libsys/getpid.s
+++ b/plat/msdos386/libsys/getpid.s
@@ -39,7 +39,7 @@ _getpid:
 	jnc .eur_dos
 	movb ah, 0x51
 	int 0x21
-	xchg bx, ax
+	xchg ebx, eax
 .eur_dos:
-	xor dx, dx
+	xor edx, edx
 	ret
diff --git a/plat/msdos386/libsys/isatty.s b/plat/msdos386/libsys/isatty.s
index bd2e195e6..7ed92fffd 100644
--- a/plat/msdos386/libsys/isatty.s
+++ b/plat/msdos386/libsys/isatty.s
@@ -16,22 +16,22 @@
 
 .define _isatty
 _isatty:
-	mov bx, sp
-	mov bx, 2(bx)
-	mov ax, 0x4400
+	mov ebx, sp
+	mov ebx, 4(ebx)
+	o16 mov ax, 0x4400
 	int 0x21
 	jc error
 	testb dl, dl
 	jz not_tty
-	mov ax, 1
+	mov eax, 1
 	ret
 not_tty:
 	mov (_errno), 25		! ENOTTY
 not_tty_2:
-	xor ax, ax
+	xor eax, eax
 	ret
 error:
-	push ax
+	push eax
 	call __sys_seterrno
-	pop cx
+	pop ecx
 	jmp not_tty_2
diff --git a/plat/msdos386/libsys/rename.s b/plat/msdos386/libsys/rename.s
index edb14266b..1550b73db 100644
--- a/plat/msdos386/libsys/rename.s
+++ b/plat/msdos386/libsys/rename.s
@@ -16,6 +16,7 @@
 
 .define _rename
 _rename:
+	int 3
 	mov bx, sp
 	push di
 	mov dx, 2(bx)
diff --git a/plat/msdos386/libsys/sys_exists.s b/plat/msdos386/libsys/sys_exists.s
index e282ef687..8786f4c2a 100644
--- a/plat/msdos386/libsys/sys_exists.s
+++ b/plat/msdos386/libsys/sys_exists.s
@@ -16,6 +16,7 @@
 
 .define __sys_exists
 __sys_exists:
+	int 3
 	mov bx, sp
 	mov dx, 2(bx)
 	mov ax, 0x4300
diff --git a/plat/msdos386/libsys/sys_getdate.s b/plat/msdos386/libsys/sys_getdate.s
index 41d69fc85..eeb118923 100644
--- a/plat/msdos386/libsys/sys_getdate.s
+++ b/plat/msdos386/libsys/sys_getdate.s
@@ -16,6 +16,7 @@
 
 .define __sys_getdate
 __sys_getdate:
+	int 3
 	movb ah, 0x2a
 	int 0x21
 	mov bx, sp
diff --git a/plat/msdos386/libsys/sys_gettime.s b/plat/msdos386/libsys/sys_gettime.s
index d062fad8d..d3ea65847 100644
--- a/plat/msdos386/libsys/sys_gettime.s
+++ b/plat/msdos386/libsys/sys_gettime.s
@@ -16,6 +16,7 @@
 
 .define __sys_gettime
 __sys_gettime:
+	int 3
 	movb ah, 0x2c
 	int 0x21
 	mov bx, sp
diff --git a/plat/msdos386/libsys/sys_isopen.s b/plat/msdos386/libsys/sys_isopen.s
index 5ce49ed45..5da95a44e 100644
--- a/plat/msdos386/libsys/sys_isopen.s
+++ b/plat/msdos386/libsys/sys_isopen.s
@@ -17,6 +17,7 @@
 
 .define __sys_isopen
 __sys_isopen:
+	int 3
 	mov bx, sp
 	mov bx, 2(bx)
 	mov ax, 0x4400
diff --git a/plat/msdos386/libsys/sys_isreadyr.s b/plat/msdos386/libsys/sys_isreadyr.s
index b5d0800ee..9a400859d 100644
--- a/plat/msdos386/libsys/sys_isreadyr.s
+++ b/plat/msdos386/libsys/sys_isreadyr.s
@@ -17,6 +17,7 @@
 
 .define __sys_isreadyr
 __sys_isreadyr:
+	int 3
 	mov bx, sp
 	mov ax, 0x4406
 	mov bx, 2(bx)
diff --git a/plat/msdos386/libsys/sys_rawcreat.s b/plat/msdos386/libsys/sys_rawcreat.s
index 67bc34e59..0d765a27d 100644
--- a/plat/msdos386/libsys/sys_rawcreat.s
+++ b/plat/msdos386/libsys/sys_rawcreat.s
@@ -16,6 +16,7 @@
 
 .define __sys_rawcreat
 __sys_rawcreat:
+	int 3
 	movb ah, 0x3c
 	mov bx, sp
 	mov dx, 2(bx)
diff --git a/plat/msdos386/libsys/sys_rawlseek.s b/plat/msdos386/libsys/sys_rawlseek.s
index 992c4c655..06a7783a7 100644
--- a/plat/msdos386/libsys/sys_rawlseek.s
+++ b/plat/msdos386/libsys/sys_rawlseek.s
@@ -16,6 +16,7 @@
 
 .define __sys_rawlseek
 __sys_rawlseek:
+	int 3
 	movb ah, 0x42
 	mov bx, sp
 	mov dx, 4(bx)
diff --git a/plat/msdos386/libsys/sys_rawopen.s b/plat/msdos386/libsys/sys_rawopen.s
index 301e2fded..5fae1bba4 100644
--- a/plat/msdos386/libsys/sys_rawopen.s
+++ b/plat/msdos386/libsys/sys_rawopen.s
@@ -16,6 +16,7 @@
 
 .define __sys_rawopen
 __sys_rawopen:
+	int 3
 	movb ah, 0x3d
 	mov bx, sp
 	mov dx, 2(bx)
diff --git a/plat/msdos386/libsys/sys_rawrw.s b/plat/msdos386/libsys/sys_rawread.s
similarity index 56%
rename from plat/msdos386/libsys/sys_rawrw.s
rename to plat/msdos386/libsys/sys_rawread.s
index a306d6528..d7c14aa54 100644
--- a/plat/msdos386/libsys/sys_rawrw.s
+++ b/plat/msdos386/libsys/sys_rawread.s
@@ -12,7 +12,13 @@
 
 .sect .text
 
-! Read/write bytes to/to a file descriptor.  These routines do not do any
+.extern pmode_ds
+.extern pmode_cs
+.extern rmode
+.extern transfer_buffer_ptr
+.extern interrupt_ptr
+
+! Read bytes from a file descriptor.  These routines do not do any
 ! translation between CRLF and LF line endings.
 !
 ! Note that, for MS-DOS, a raw "write" request of zero bytes will truncate
@@ -20,14 +26,5 @@
 
 .define __sys_rawread
 __sys_rawread:
-	movb ah, 0x3f
-	.data1 0x3d			! eat up the next instruction
-.define __sys_rawwrite
-__sys_rawwrite:
-	movb ah, 0x40
-	mov bx, sp
-	mov dx, 4(bx)
-	mov cx, 6(bx)
-	mov bx, 2(bx)
-	int 0x21
-	jmp .sys_axret
+	int 3
+
diff --git a/plat/msdos386/libsys/sys_rawwrite.s b/plat/msdos386/libsys/sys_rawwrite.s
new file mode 100644
index 000000000..e13e5e52c
--- /dev/null
+++ b/plat/msdos386/libsys/sys_rawwrite.s
@@ -0,0 +1,70 @@
+#
+! $Source$
+! $State$
+! $Revision$
+
+! Declare segments (the order is important).
+
+.sect .text
+.sect .rom
+.sect .data
+.sect .bss
+
+.sect .text
+
+.extern pmode_ds
+.extern pmode_cs
+.extern rmode
+.extern transfer_buffer_ptr
+.extern interrupt_ptr
+
+! Write bytes to/to a file descriptor.  These routines do not do any
+! translation between CRLF and LF line endings.
+!
+! Note that, for MS-DOS, a raw "write" request of zero bytes will truncate
+! (or extend) the file to the current file position.
+
+.define __sys_rawwrite
+__sys_rawwrite:
+	enter 4, 0
+amount_transferred = -1*4
+file_handle = 2*4
+read_buffer = 3*4
+amount_to_write = 4*4
+
+	mov es, (pmode_ds)
+	mov amount_transferred(ebp), 0
+
+mainloop:
+	mov ecx, 32*1024
+	cmp amount_to_write(ebp), ecx
+	jge 2f
+	mov ecx, amount_to_write(ebp)
+	test ecx, ecx
+	jz exit
+2:
+	! Copy ecx bytes into the transfer buffer.
+
+	push ecx
+	mov esi, read_buffer(ebp)
+	movzx edi, (transfer_buffer_ptr)
+	rep movsb
+	pop ecx
+
+	movb ah, 0x40
+	o16 mov dx, (transfer_buffer_ptr)
+	o16 mov bx, file_handle(ebp)
+	or ebx, 0x210000
+	callf (interrupt_ptr)
+	movzx eax, ax
+
+	add read_buffer(ebp), eax
+	add amount_transferred(ebp), eax
+	sub amount_to_write(ebp), eax
+	jmp mainloop
+
+exit:
+	mov eax, amount_to_write(ebp)
+	leave
+	ret
+
diff --git a/plat/msdos386/libsys/sys_xret.s b/plat/msdos386/libsys/sys_xret.s
index 57f407ef4..20c2b454b 100644
--- a/plat/msdos386/libsys/sys_xret.s
+++ b/plat/msdos386/libsys/sys_xret.s
@@ -13,29 +13,38 @@
 .sect .text
 
 ! .sys_zret: if the carry flag is set, then set `errno' from the DOS error
-! code in ax, and return a shortword -1.  If the carry flag is clear, just
-! return a shortword zero.
+! code in ax, and return an int -1.  If the carry flag is clear, just
+! return an int zero.
 !
 ! .sys_axret: if the carry flag is set, then set `errno' from the DOS error
 ! code in ax, and return a shortword -1.  If the carry flag is clear, just
 ! return ax as a return value.
 !
-! .sys_dxaxret: same as .sys_axret, but return a longword -1 or dx:ax as a
-! return value.
+! .sys_dxaxret: same as .sys_axret, but return -1 or dx:ax as a return value.
 
 .extern .sys_zret
 .extern .sys_axret
 .extern .sys_dxaxret
 .sys_zret:
 	jc error
-	xor ax, ax
+	xor eax, eax
 no_error:
 	ret
+
 .sys_axret:
-.sys_dxaxret:
-	jnc no_error
-error:
-	push ax
-	call __sys_seterrno
-	pop cx
+	jc error
+	ret
+
+.sys_dxaxret:
+	jc error
+	shl edx, 16
+	o16 mov dx, ax
+	mov eax, edx
+	ret
+
+error:
+	movzx eax, ax
+	push eax
+	call __sys_seterrno
+	pop ecx
 	ret
diff --git a/plat/msdos386/stub.s b/plat/msdos386/stub.s
index 863d5d8f7..03887ec5d 100644
--- a/plat/msdos386/stub.s
+++ b/plat/msdos386/stub.s
@@ -210,15 +210,17 @@ exe_start:
 
     ! Jump to the new segment and enter 32-bit mode!
 
-    mov ax, (psegcs)
-    mov bx, (psegds)
-    mov cx, (rseg)
+    o32 movzx eax, (psegcs)
+    o32 movzx ebx, (psegds)
+    o32 movzx ecx, (rseg)
     o32 mov edx, realloc
+    o32 mov esi, interruptf
+    o32 mov edi, transfer_buffer
     push es
     pop ds
     push es
     push 0
-    retf
+    retf ! 19b
 
     ! ALL CODE ABOVE THIS POINT DISCARDED ON ENTRY TO 32-BIT CODE
     ! (it's reused as the 16-bit stack)
@@ -271,6 +273,7 @@ realloc:
     jc bad_dpmi
     
     popa
+    o32 mov eax, (pmemlen)
     cli                         ! atomically switch stacks back
     mov ss, (dpmi_ss)
     o32 mov esp, (dpmi_ebp)
@@ -309,10 +312,15 @@ exit_with_error:
 
     ! Simulate DOS interrupt.
 int21:
+    o32 or ebx, 0x210000
+    ! Simulate interrupt in the high half of ebx.
+interrupt:
     mov (dpmi_eax), ax
     mov (dpmi_ebx), bx
     mov (dpmi_ecx), cx
     mov (dpmi_edx), dx
+    mov (dpmi_esi), si
+    mov (dpmi_edi), di
     mov ax, (rseg)
     mov (dpmi_ds), ax
     push es
@@ -324,16 +332,28 @@ int21:
     pop es
     mov di, dpmi_edi
     mov ax, 0x300
-    mov bx, 0x21
+    o32 shr ebx, 16
     int 0x31                    ! simulate DOS interrupt
     pop ds
     pop es
+    pushf
     mov ax, (dpmi_eax)
     mov bx, (dpmi_ebx)
     mov cx, (dpmi_ecx)
     mov dx, (dpmi_edx)
+    mov si, (dpmi_esi)
+    mov di, (dpmi_edi)
+    popf
     ret
 
+! Far call wrapper around interrupt.
+interruptf:
+    push ds
+    cseg mov ds, (psegds)
+    call interrupt
+    pop ds
+    o32 retf
+
 bad_dpmi_msg:
     .asciz "DPMI error during setup"
 no_file_msg: