init DOS_PSP with PGROUP [#172][skip CI]

We used to have DOS_PSP=cs, but now we have ELF headers in DOS space.
And as we use tiny model, cs:0 inevitably points to them.
To not overwrite them with PSP, move DOS_PSP var to kernel.asm and
initialize it with PGROUP.
This patch makes PGROUP no longer equal to other segments, so this
needs a proper segelf support. This can't be done in a master branch
where we relocate by a hard-coded value only (0x60).
So this patch shows a clear advantage of the segelf scheme usage.

Note that even the part of this patch, eg aligning PSP section to
0x100, cannot be back-ported to master because there we store PSP
at cs:0 regardless of the linker script, and overwrite whatever
is there. And there are the ELF headers now, since commit 95432f8.
Which probably shouldn't have been back-ported to master as a start.

fdpp/kernel.ld

@@ -2,20 +2,23 @@
 
 /* these GROUPs play the same role as GROUPS (segments) in OMF */
 /* TINY model, all segments are equal */
-_PGROUP = 0;
 _LGROUP = 0;
 _DGROUP = 0;
 _IGROUP = 0;
 
 SECTIONS
   {
+    . = SIZEOF_HEADERS;
+    . = ALIGN(0x100);
     /* Target PSP section.  */
-    .ptext SIZEOF_HEADERS : {
+    .ptext : {
+		_PGROUP = .;
 		*(PSP)
+		. = ALIGN(0x100);
     }
 
     /* Target low data+text sections.  */
-    .ltext 0x200 : {
+    .ltext : {
 		*(_IRQTEXT)
 		*(_LOWTEXT)
 		*(_IO_TEXT)

kernel/glob_asm.h

@@ -1,5 +1,6 @@
 /* grep "ASM " globals.h | grep "extern" | grep ";" | sed 's/extern \(.\+\) ASM \(.\+\);/__ASM(\1, \2) SEMIC/' */
 
+__ASM(UWORD, DOS_PSP) SEMIC
 __ASM(UWORD, NetBios) SEMIC
 __ASM(BYTE *, net_name) SEMIC
 __ASM(BYTE, net_set_count) SEMIC

kernel/kernel.asm

@@ -778,6 +778,8 @@ begin_hma:
 ; to minimize relocations
                 global _DGROUP_
 _DGROUP_        dw DGROUP
+                global _DOS_PSP
+_DOS_PSP        dw PGROUP
 
 %ifdef WATCOM
 ;               32 bit multiplication + division

kernel/main.c

@@ -73,8 +73,6 @@ __segment DosTextSeg = 0;
 
 #endif
 
-UWORD DOS_PSP;
-
 ASMREF(struct lol) LoL = __ASMADDR(DATASTART);
 struct _bprm bprm;
 #define TEXT_SIZE (_InitTextEnd - _HMATextStart)
@@ -91,7 +89,6 @@ VOID ASMCFUNC FreeDOSmain(void)
   DosDataSeg = (__segment) & DATASTART;
   DosTextSeg = (__segment) & prn_dev;
 #endif
-  DOS_PSP = _CS;
   /* back up kernel code to the top of ram */
   lpTop = MK_FP(_SS - (TEXT_SIZE + 15) / 16, 0);
   fmemcpy(lpTop, _HMATextStart, TEXT_SIZE);