plat/msdos386: fix startup code so that it works under dosemu2

- do not try to store data through code segment selectors
 - set each segment limit to segment size - 1, not segment size
 - get PSP segment while still in real/V86 mode (DPMI host
   might choose to return PM selector for int 0x21, ah = 0x62)
 - other minor fixes

plat/msdos386/boot.s

@@ -21,23 +21,24 @@ begtext:
     !   ax: pmode code segment of stub
     !   bx: pmode data segment of stub
     !   cx: rmode segment of stub
-    !   dx: pointer to realloc routine (in stub)e
+    !   dx: pointer to realloc routine (in stub)
     !   si: pointer to interrupt routine (in stub)
     !   di: pointer to transfer buffer (in stub)
+    !   bp: rmode PSP segment
 
     ! Resize the segment to include the BSS.
 
-    o16 cseg mov (realloc_ptr+4), ax
+    o16 mov (realloc_ptr+4), ax
-    cseg mov (realloc_ptr+0), edx
+    mov (realloc_ptr+0), edx
-    cseg o16 mov (pmode_cs), ax
+    o16 mov (pmode_cs), ax
-    cseg o16 mov (pmode_ds), bx
+    o16 mov (pmode_ds), bx
-    cseg o16 mov (rmode), cx
+    o16 mov (rmode), cx
-    cseg o16 mov (interrupt_ptr+4), ax
+    o16 mov (interrupt_ptr+4), ax
-    cseg mov (interrupt_ptr+0), esi
+    mov (interrupt_ptr+0), esi
-    cseg o16 mov (transfer_buffer_ptr), di
+    o16 mov (transfer_buffer_ptr), di
 
     mov eax, __end
-    cseg callf (realloc_ptr)
+    callf (realloc_ptr)
 
     ! Clear BSS.
 
@@ -71,7 +72,8 @@ begtext:
     xor edx, edx
     o16 mov ax, 0x0007
     int 0x31                    ! set base address to 0
-    o16 mov cx, 0x0010
+    o16 dec dx
+    o16 mov cx, 0x000f
     o16 mov ax, 0x0008
     int 0x31                    ! set limit to 1MB
 
@@ -84,9 +86,7 @@ begtext:
 
     ! Locate the PSP.
     
-    movb ah, 0x62
+    movzx esi, bp
-    int 0x21
-    movzx esi, bx
     shl esi, 4                  ! convert to linear address
 
     ! Copy the whole thing into 32-bit memory.

plat/msdos386/stub.s

@@ -127,7 +127,7 @@ exe_start:
     mov cx, 1
     int 0x31                    ! allocate LDT
     jc bad_dpmi
-    mov es, ax
+    mov fs, ax
     mov (psegcs32), ax
 
     mov cx, (pmemlen+0)
@@ -140,12 +140,12 @@ exe_start:
 
     mov dx, cx
     mov cx, bx
-    mov bx, es
+    mov bx, fs
     mov ax, 0x0007
     int 0x31                    ! set segment base address
     jc bad_dpmi
 
-    mov bx, es
+    mov bx, fs
     mov dx, (pmemlen+0)
     mov cx, (pmemlen+2)
     mov ax, 0x0008
@@ -155,23 +155,23 @@ exe_start:
     and cx, 3
     shl cx, 5
     or cx, 0xc09b               ! 32-bit, big, code, non-conforming, readable
-    mov bx, es
+    mov bx, fs
     mov ax, 0x0009
     int 0x31                    ! set descriptor access rights
 
     ! Allocate the data segment (as a simple clone of the code segment). (10e)
 
     mov ax, 0x000a
-    mov bx, es
+    mov bx, fs
     int 0x31
-    mov fs, ax
+    mov es, ax
     mov (psegds32), ax
 
     mov cx, ax
     and cx, 3
     shl cx, 5
     or cx, 0xc093               ! 32-bit, big, data, r/w, expand-up
-    mov bx, fs
+    mov bx, es
     mov ax, 0x0009
     int 0x31                    ! set descriptor access rights
 
@@ -198,7 +198,7 @@ exe_start:
     o32 movzx ecx, ax           ! number of bytes read
     o32 mov esi, transfer_buffer
     cld
-    o32 rep movsb
+    rep a32 movsb
     jmp 1b
 2:
 
@@ -216,9 +216,10 @@ exe_start:
     o32 mov edx, realloc
     o32 mov esi, interruptf
     o32 mov edi, transfer_buffer
+    o32 movzx ebp, (pspseg)
     push es
     pop ds
-    push es
+    push fs
     push 0
     retf ! 19b
 
@@ -253,19 +254,21 @@ realloc:
     mov (pmemaddr+0), cx
     mov (pmemaddr+2), bx
 
-    mov bx, (psegds32)
+    mov bx, (psegcs32)
-    mov dx, (pmemlen+0)
-    mov cx, (pmemlen+2)
-    mov ax, 0x0008
-    int 0x31                    ! set ds segment limit
-    jc bad_dpmi
     mov dx, (pmemaddr+0)
     mov cx, (pmemaddr+2)
     mov ax, 0x0007
+    int 0x31                    ! set cs linear address
+    jc bad_dpmi
+    mov bx, (psegds32)
     int 0x31                    ! set ds linear address
     jc bad_dpmi
-    mov bx, (psegcs32)
+    mov dx, (pmemlen+0)
-    int 0x31                    ! set cs linear address
+    mov cx, (pmemlen+2)
+    sub dx, 1
+    sbb cx, 0
+    mov ax, 0x0008
+    int 0x31                    ! set ds segment limit
     jc bad_dpmi
     
     popa
@@ -308,6 +311,7 @@ exit_with_error:
 
     ! Simulate DOS interrupt.
 int21:
+    o32 movzx ebx, bx
     o32 or ebx, 0x210000
     ! Simulate interrupt in the high half of ebx.
 interrupt:
@@ -327,6 +331,7 @@ interrupt:
     o32 mov edi, dpmi_edi
     mov ax, 0x300
     o32 shr ebx, 16
+    xor cx, cx
     int 0x31                    ! simulate DOS interrupt
     pop es
     o32 movzx eax, (dpmi_eax)