From 246b0037a8314c3c5ebe216c386904674c683fc4 Mon Sep 17 00:00:00 2001
From: Kenneth J Davis <PerditionC@gmail.com>
Date: Sun, 8 Aug 2021 03:52:58 -0400
Subject: [PATCH] initial implementation of extended seek (0x7142 - LONG LSEEK
 with 64-bit file position)

---
 kernel/dosfns.c   | 14 +++++++++++---
 kernel/inthndlr.c | 23 ++++++++++++++++++-----
 2 files changed, 29 insertions(+), 8 deletions(-)

diff --git a/kernel/dosfns.c b/kernel/dosfns.c
index 23d9974..74a0c10 100644
--- a/kernel/dosfns.c
+++ b/kernel/dosfns.c
@@ -306,7 +306,7 @@ long DosRWSft(int sft_idx, size_t n, void FAR * bp, int mode)
   return rwblock(sft_idx, bp, n, mode);
 }
 
-COUNT SftSeek(int sft_idx, LONG new_pos, unsigned mode)
+COUNT SftSeek2(int sft_idx, LONG new_pos, unsigned mode, UDWORD * p_result)
 {
   sft FAR *s = idx_to_sft(sft_idx);
   if (FP_OFF(s) == (size_t) -1)
@@ -347,17 +347,25 @@ COUNT SftSeek(int sft_idx, LONG new_pos, unsigned mode)
   }
 
   s->sft_posit = new_pos;
+  *p_result = new_pos;
   return SUCCESS;
 }
 
+COUNT SftSeek(int sft_idx, LONG new_pos, unsigned mode)
+{
+  UDWORD result;
+  return SftSeek2(sft_idx, new_pos, mode, &result);
+}
+
 ULONG DosSeek(unsigned hndl, LONG new_pos, COUNT mode, int *rc)
 {
   int sft_idx = get_sft_idx(hndl);
+  UDWORD result;
 
   /* Get the SFT block that contains the SFT      */
-  *rc = SftSeek(sft_idx, new_pos, mode);
+  *rc = SftSeek2(sft_idx, new_pos, mode, &result);
   if (*rc == SUCCESS)
-    return idx_to_sft(sft_idx)->sft_posit;
+    return result;
   return *rc;
 }
 
diff --git a/kernel/inthndlr.c b/kernel/inthndlr.c
index 788cc5d..73dada8 100644
--- a/kernel/inthndlr.c
+++ b/kernel/inthndlr.c
@@ -952,8 +952,7 @@ dispatch:
     case 0x42:
       if (lr.AL > 2)
         goto error_invalid;
-      lrc = DosSeek(lr.BX, (LONG)((((ULONG) (lr.CX)) << 16) | lr.DX), lr.AL,
-        &rc);
+      lrc = DosSeek(lr.BX, (LONG)MK_ULONG(lr.CX, lr.DX), lr.AL, &rc);
       if (rc == SUCCESS)
       {
         lr.DX = (UWORD)(lrc >> 16);
@@ -1601,8 +1600,9 @@ lfn_findclose:
           goto unsupp;
 #endif
 #endif
+        /* EDR-DOS LFN - Long LSEEK - SET CURRENT 64-bit FILE POSITION */
+        case 0x42:
         /* Win95 LFN - get file info by handle */
-        case 0x42:  /* ??? */
         case 0xa6: {
           /* only passed to redirector supported for now */
           iregs saved_r;
@@ -1621,8 +1621,11 @@ lfn_findclose:
             rc = DE_INVLDHNDL;
             goto error_exit;
           }
-          if (!(s->sft_flags & SFT_FSHARED))
-            goto unsupp;    /* unsupported on local fs yet */
+          if (!(s->sft_flags & SFT_FSHARED)) {
+            if (lr.AL != 0x42) {
+              goto unsupp;    /* unsupported on local fs yet */
+            }
+          }
           /* call to redirector */
           saved_r = *r;
           r->ES = FP_SEG(s);
@@ -2201,6 +2204,16 @@ VOID ASMCFUNC int2F_12_handler(struct int2f12regs FAR *pr)
       rc = DosClose(r.BX);
       goto short_check;
 
+    case 0x42:                 /* 64-bit move file pointer */
+    {
+      /* r.(DS:DX) points to 64-bit file position instead of r.(CX:DX) being 32-bit file position */
+      UWORD FAR *filepos = MK_FP(r.DS, r.DX);
+      if (*(filepos+1) != 0) /* we currently only handle lower 32 bits, upper 32 bits must be 0 */
+        goto error_invalid;
+      r.BP = (UWORD)r.CL;
+      r.CX = *filepos;
+      /* fall through to 32-bit move file pointer (0x28) */
+    }
     case 0x28:                 /* move file pointer */
       /*
        * RBIL says: "sets user stack frame pointer to dummy buffer,