plat/msdos386: fix some issues in libsys code

- ensure es = ds = ss upon exit of each C runtime function
 - clear upper 16 bits of ebx before setting them to 0x0021,
   when invoking interrupt_ptr to simulate a RM int 0x21
 - make _sys_exists use the transfer buffer (which it needs)
 - make _sys_rawread properly handle an end-of-file read
   (zero bytes read)
 - make argument to _sys_seterrno short --- after a failed
   int 0x21 call, only the lower 16 bits of eax hold the MS-DOs
   error code
 - _sys_rawlseek accepts only 3 longword arguments, not 4
   (the offset is only 1 longword)
 - other minor fixes

plat/msdos/libsys/sys_seterrno.c

@@ -35,7 +35,7 @@ static const signed char err_map[] =
  * Map an MS-DOS 2+ system error code to an `errno' value and store that in
  * `errno'.  Return a longword -1.
  */
-long _sys_seterrno(unsigned dos_err)
+long _sys_seterrno(unsigned short dos_err)
 {
 	if (dos_err < sizeof(err_map) / sizeof(err_map[0]))
 		errno = err_map[dos_err];

plat/msdos386/libsys/getpid.s

@@ -38,8 +38,9 @@ _getpid:
 	int 0x21
 	jnc .eur_dos
 	movb ah, 0x51
-	int 0x21
+	mov ebx, 0x210000
+	callf (interrupt_ptr)
 	xchg ebx, eax
 .eur_dos:
-	xor edx, edx
+	movzx eax, ax
 	ret

plat/msdos386/libsys/rename.s

@@ -23,9 +23,9 @@ _rename:
 	! Source filename.
 
 	mov esi, 4(ebx)
-    movzx edi, (transfer_buffer_ptr)
-    mov es, (pmode_ds)
-    cld
+	movzx edi, (transfer_buffer_ptr)
+	mov es, (pmode_ds)
+	cld
 1:
 	lodsb
 	stosb
@@ -47,8 +47,10 @@ _rename:
 	o16 mov dx, (transfer_buffer_ptr)
 	o16 mov di, ax
 	movb ah, 0x56
-	or ebx, 0x210000
+	mov ebx, 0x210000
 	callf (interrupt_ptr)
 	pop edi
 	pop esi
+	push ss
+	pop es
 	jmp .sys_zret

plat/msdos386/libsys/sys_exists.s

@@ -16,10 +16,22 @@
 
 .define __sys_exists
 __sys_exists:
-	mov ebx, esp
-	mov edx, 4(ebx)
+	push esi
+	mov esi, 4+4(esp)
+	movzx edi, (transfer_buffer_ptr)
+	mov edx, edi
+	mov es, (pmode_ds)
+	cld
+1:
+	lodsb
+	stosb
+	testb al, al
+	jnz 1b
 	mov eax, 0x4300
-	int 0x21
+	mov ebx, 0x210000
+	callf (interrupt_ptr)
+	push ss
+	pop es
 	sbb eax, eax
 	inc eax
 	ret

plat/msdos386/libsys/sys_gettime.s

@@ -16,7 +16,6 @@
 
 .define __sys_gettime
 __sys_gettime:
-	int 3
 	movb ah, 0x2c
 	int 0x21
 	mov ebx, esp

plat/msdos386/libsys/sys_isopen.s

@@ -17,8 +17,7 @@
 
 .define __sys_isopen
 __sys_isopen:
-	mov ebx, esp
-	mov ebx, 4(bx)
+	mov ebx, 4(esp)
 	mov eax, 0x4400
 	int 0x21
 	sbb eax, eax

plat/msdos386/libsys/sys_rawcreat.s

@@ -36,10 +36,12 @@ __sys_rawcreat:
 	movb ah, 0x3c
 	movzx edx, (transfer_buffer_ptr)
 	movb al, 8(ebx)
-	or ebx, 0x210000
+	mov ebx, 0x210000
 	callf (interrupt_ptr)
 
 	pop edi
 	pop esi
+	push ss
+	pop es
 	jmp .sys_axret
 

plat/msdos386/libsys/sys_rawlseek.s

@@ -18,9 +18,10 @@
 __sys_rawlseek:
 	movb ah, 0x42
 	mov ebx, esp
-	mov edx, 8(bx)
-	mov ecx, 12(bx)
-	movb al, 16(bx)
-	mov ebx, 4(bx)
+	mov edx, 8(ebx)
+	mov ecx, edx
+	shr ecx, 16
+	movb al, 12(ebx)
+	mov ebx, 4(ebx)
 	int 0x21
 	jmp .sys_dxaxret

plat/msdos386/libsys/sys_rawopen.s

@@ -37,9 +37,11 @@ __sys_rawopen:
 	movb ah, 0x3d
 	o16 mov dx, (transfer_buffer_ptr)
 	movb al, 8(ebx)
-	or ebx, 0x210000
+	mov ebx, 0x210000
 	callf (interrupt_ptr)
 
 	pop edi
 	pop esi
+	push ss
+	pop es
 	jmp .sys_axret

plat/msdos386/libsys/sys_rawread.s

@@ -38,8 +38,9 @@ amount_to_read = 4*4
 
 	movb ah, 0x3f
 	o16 mov dx, (transfer_buffer_ptr)
+	movzx esi, dx
+	mov ebx, 0x210000
 	o16 mov bx, file_handle(ebp)
-	or ebx, 0x210000
 	callf (interrupt_ptr)
 	jnc success
 
@@ -53,9 +54,10 @@ success:
 
 	! Copy eax bytes out of the transfer buffer.
 
-	push eax
+	movzx eax, ax
 	mov ecx, eax
-	movzx esi, (transfer_buffer_ptr)
+	jcxz exit
+	push eax
 	mov edi, write_buffer(ebp)
 	mov es, (pmode_ds)
 	cld
@@ -69,6 +71,8 @@ success:
 exit:
 	pop edi
 	pop esi
+	push ss
+	pop es
 	leave
 	ret
 

plat/msdos386/libsys/sys_rawwrite.s

@@ -48,8 +48,8 @@ amount_to_write = 4*4
 
     movb ah, 0x40
     o16 mov dx, (transfer_buffer_ptr)
+    mov ebx, 0x210000
     o16 mov bx, file_handle(ebp)
-    or ebx, 0x210000
     callf (interrupt_ptr)
     jnc exit
 
@@ -59,6 +59,8 @@ amount_to_write = 4*4
 exit:
     pop edi
     pop esi
+    push ss
+    pop es
     leave
     ret
 

plat/msdos386/libsys/sys_xret.s

@@ -20,7 +20,8 @@
 ! 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 -1 or dx:ax as a return value.
+! .sys_dxaxret: same as .sys_axret, but return -1 or eax := dx:ax as a
+! return value.
 
 .extern .sys_zret
 .extern .sys_axret

plat/msdos386/libsys/unlink.s

@@ -34,11 +34,13 @@ _unlink:
 
 	! Make the DOS call.
 
-    movzx edi, (transfer_buffer_ptr)
+    mov dx, (transfer_buffer_ptr)
 	movb ah, 0x41
 	or ebx, 0x210000
 	callf (interrupt_ptr)
 
 	pop edi
 	pop esi
+	push ss
+	pop es
 	jmp .sys_zret