From 461ceab52ae6252be3e74ac31cf59dd476f44513 Mon Sep 17 00:00:00 2001
From: Stas Sergeev <stsp2@yandex.ru>
Date: Wed, 1 Apr 2026 11:53:43 +0300
Subject: [PATCH] hmamgr: implement topdown allocations [fixes #292]

---
 kernel/hmamgr.c   | 23 ++++++++++++++++-------
 kernel/inthndlr.c | 10 +++-------
 kernel/proto.h    |  2 +-
 3 files changed, 20 insertions(+), 15 deletions(-)

diff --git a/kernel/hmamgr.c b/kernel/hmamgr.c
index c8deec41..79f5a5c7 100644
--- a/kernel/hmamgr.c
+++ b/kernel/hmamgr.c
@@ -34,7 +34,8 @@ static hmcb init_mcb = { .signature = HMCB_SIG, };
 #define FIRST_MCB 0x10
 
 #define nxtMCBsize(mcb,size) MK_FP(FP_SEG(mcb), FP_OFF(mcb) + (size) + 16)
-#define nxtMCB(mcb) MK_FP(FP_SEG(mcb), mcb->next)
+#define nxtMCBsizeTd(mcb,size) MK_FP(FP_SEG(mcb), FP_OFF(mcb) + (mcb)->size - (size))
+#define nxtMCB(mcb) MK_FP(FP_SEG(mcb), (mcb)->next)
 
 #define mcbFree(mcb) ((mcb)->owner == 0)
 #define mcbValid(mcb) ( ((mcb)->signature == HMCB_SIG) )
@@ -99,10 +100,10 @@ STATIC COUNT joinMCBs(UWORD off)
    size is the minimum size of the block to search for,
    even if mode == LARGEST.
  */
-COUNT DosHMAAlloc(UWORD size, UWORD *off)
+COUNT DosHMAAlloc(UWORD size, BOOL topdown, UWORD *off)
 {
-  REG hmcb FAR *p;
-  REG hmcb FAR *q = NULL;
+  hmcb FAR *p;
+  hmcb FAR *q = NULL;
   hmcb FAR *foundSeg;
   hmcb FAR *biggestSeg;
 
@@ -156,13 +157,16 @@ COUNT DosHMAAlloc(UWORD size, UWORD *off)
     /* foundSeg := pointer to allocated block
        p := pointer to MCB that will form the rest of the block
      */
-    p = nxtMCBsize(foundSeg, size);
+    p = topdown ? nxtMCBsizeTd(foundSeg, size) : nxtMCBsize(foundSeg, size);
 
     fd_prot_mem(p, sizeof(*p), FD_MEM_NORMAL);
     /* initialize stuff because p > foundSeg  */
     *p = init_mcb;
     p->next = foundSeg->next;
-    p->size = foundSeg->size - size - 16;
+    if (topdown)
+      p->size = size;
+    else
+      p->size = foundSeg->size - size - 16;
     fd_mark_mem(p, sizeof(*p), FD_MEM_READONLY);
     fd_prot_mem(foundSeg, sizeof(*foundSeg), FD_MEM_NORMAL);
     foundSeg->next = mcb2off(p);
@@ -173,8 +177,13 @@ COUNT DosHMAAlloc(UWORD size, UWORD *off)
     p->owner = 0;        /* unused */
     fd_prot_mem(p, sizeof(*p), FD_MEM_READONLY);
 
-    foundSeg->size = size;
+    if (topdown)
+      foundSeg->size -= size + 16;
+    else
+      foundSeg->size = size;
     fd_prot_mem(foundSeg, sizeof(*foundSeg), FD_MEM_READONLY);
+    if (topdown)
+      foundSeg = p;
   }
 
   /* Already initialized:
diff --git a/kernel/inthndlr.c b/kernel/inthndlr.c
index 7292b807..c07b5738 100644
--- a/kernel/inthndlr.c
+++ b/kernel/inthndlr.c
@@ -1861,7 +1861,7 @@ VOID ASMCFUNC int2F_12_handler(struct int2f12regs FAR *regs)
       r.BX = (r.BX + 0xf) & 0xfff0;
       if (r.BX > avail)
         r.BX = avail;
-      rc = DosHMAAlloc(r.BX, &r.DI);
+      rc = DosHMAAlloc(r.BX, 0, &r.DI);
       break;
     case 3:
       if (!HMAclaimed)
@@ -1876,14 +1876,10 @@ VOID ASMCFUNC int2F_12_handler(struct int2f12regs FAR *regs)
       switch (r.DL)
       {
         case 0:  // alloc
+        case 1:
           /* align to para */
           r.BX = (r.BX + 0xf) & 0xfff0;
-          rc = DosHMAAlloc(r.BX, &r.DI);
-          break;
-        case 1:  // resize
-          /* align to para */
-          r.BX = (r.BX + 0xf) & 0xfff0;
-          rc = DosHMAChange(r.DI, r.BX);
+          rc = DosHMAAlloc(r.BX, r.DL, &r.DI);
           break;
         case 2:  // free
           rc = DosHMAFree(r.DI);
diff --git a/kernel/proto.h b/kernel/proto.h
index 9566bdae..45633af8 100644
--- a/kernel/proto.h
+++ b/kernel/proto.h
@@ -251,7 +251,7 @@ void DosUmbLink(unsigned n);
 VOID mcb_print(__FAR(mcb) mcbp);
 
 COUNT DosHMAQuery(UWORD *off, UWORD *avail);
-COUNT DosHMAAlloc(UWORD size, UWORD *off);
+COUNT DosHMAAlloc(UWORD size, BOOL topdown, UWORD *off);
 COUNT DosHMAFree(UWORD off);
 COUNT DosHMAChange(UWORD para, UWORD size);
 COUNT DosHMACheck(void);