From c1904b50ced20d2423d5134e95fa1a174375a6a5 Mon Sep 17 00:00:00 2001
From: Lars Brinkhoff <lars@nocrew.org>
Date: Mon, 2 Jun 2025 12:57:07 +0200
Subject: [PATCH] Imlac: Model MSB and LSB accumulators.

---
 imlac/imlac_dp.c | 81 +++++++++++++++++++++++++++++++-----------------
 1 file changed, 52 insertions(+), 29 deletions(-)

diff --git a/imlac/imlac_dp.c b/imlac/imlac_dp.c
index 1fbf5fea..dc928f1e 100644
--- a/imlac/imlac_dp.c
+++ b/imlac/imlac_dp.c
@@ -35,7 +35,8 @@ static uint16 SP = 0;
 static uint16 ON = 0;
 static uint16 HALT = 0;
 static uint16 MODE = 0;
-static uint16 XA, YA;
+static uint16 XMSB, YMSB;
+static uint16 XLSB, YLSB;
 static uint16 SCALE = 2;
 static uint16 BLOCK = 0;
 static uint16 MIT8K;
@@ -68,8 +69,10 @@ static REG dp_reg[] = {
   { ORDATAD (MODE, MODE, 1, "Display mode") },
   { BRDATAD (DT, DT, 8, 16, 8, "Return address stack") },
   { ORDATAD (SP, SP, 3, "Stack pointer") },
-  { ORDATAD (XA, XA, 11, "X accumulator") },
-  { ORDATAD (YA, YA, 11, "Y accumulator") },
+  { ORDATAD (XMSB, XMSB, 7, "X accumulator MSB") },
+  { ORDATAD (XLSB, XLSB, 5, "X accumulator LSB") },
+  { ORDATAD (YMSB, YMSB, 7, "Y accumulator MSB") },
+  { ORDATAD (YLSB, YLSB, 5, "Y accumulator LSB") },
   { ORDATAD (SCALE, SCALE, 3, "Scale") },
   { ORDATAD (BLOCK, BLOCK, 3, "Block") },
   { ORDATAD (MIT8K, MIT8K, 1, "MIT 8K addressing") },
@@ -167,6 +170,25 @@ dp_iot (uint16 insn, uint16 AC)
   return AC;
 }
 
+static uint16 deflect (uint16 msb, uint16 lsb)
+{
+  return (msb << 5) + lsb;
+}
+
+static void increment (uint16 *msb, uint16 *lsb, uint16 x)
+{
+  *lsb += SCALE * x;
+  *msb += *lsb >> 5;
+  *lsb &= 037;
+}
+
+static void decrement (uint16 *msb, uint16 *lsb, uint16 x)
+{
+  *lsb -= SCALE * x;
+  *msb += ((int16)*lsb) >> 5;
+  *lsb &= 037;
+}
+
 static t_stat
 dp_opr(uint16 insn)
 {
@@ -204,7 +226,7 @@ dp_opr(uint16 insn)
   }
   if (insn & 00020) { /* DDSP */
     sim_debug (DBG, &dp_dev, "DDSP ");
-    crt_point (XA, YA);
+    crt_point (deflect (XMSB, XLSB), deflect (YMSB, YLSB));
   }
   if (insn & 00040) { /* DRJM */
     sim_debug (DBG, &dp_dev, "DRJM ");
@@ -215,19 +237,19 @@ dp_opr(uint16 insn)
   }
   if (insn & 00100) { /* DDYM */
     sim_debug (DBG, &dp_dev, "DDYM ");
-    YA -= 040;
+    YMSB--;
   }
   if (insn & 00200) { /* DDXM */
     sim_debug (DBG, &dp_dev, "DDXM ");
-    XA -= 040;
+    XMSB--;
   }
   if (insn & 00400) { /* DIYM */
     sim_debug (DBG, &dp_dev, "DIYM ");
-    YA += 040;
+    YMSB++;
   }
   if (insn & 01000) { /* DIXM */
     sim_debug (DBG, &dp_dev, "DIXM ");
-    XA += 040;
+    XMSB++;
   }
   if (insn & 02000) { /* DHVC */
     sim_debug (DBG, &dp_dev, "DHVC ");
@@ -296,7 +318,7 @@ dp_opt (uint16 insn)
 static void
 dp_inc_vector (uint16 byte)
 {
-  uint16 x1 = XA, y1 = YA;
+  uint16 x1 = deflect (XMSB, XLSB), y1 = deflect (YMSB, YLSB);
   uint16 dx, dy;
 
   if (byte == 0200) {
@@ -311,19 +333,18 @@ dp_inc_vector (uint16 byte)
     sim_debug (DBG, &dp_dev, "%o", byte & 3);
   }
 
-  dx = SCALE * ((byte >> 3) & 3);
-  dy = SCALE * (byte & 3);
+  dx = (byte >> 3) & 3;
+  dy = byte & 3;
   if (byte & 040)
-    XA -= dx;
+    decrement (&XMSB, &XLSB, dx);
   else
-    XA += dx;
+    increment (&XMSB, &XLSB, dx);
   if (byte & 4)
-    YA -= dy;
+    decrement (&YMSB, &YLSB, dy);
   else
-    YA += dy;
+    increment (&YMSB, &YLSB, dy);
   if (byte & 0100)
-    crt_line (x1, y1, XA, YA);
-
+    crt_line (x1, y1, deflect (XMSB, XLSB), deflect (YMSB, YLSB));
 }
 
 static void
@@ -347,15 +368,15 @@ dp_inc_escape (uint16 byte)
       sim_debug (DBG, &dp_dev, "stack underflow");
   }
   if (byte & 020)
-    XA += 040;
+    XMSB++;
   if (byte & 010)
-    XA &= 03740;
+    XLSB = 0;
   if (byte & 4) /* Enter PPM mode. */
     ;
   if (byte & 2)
-    YA += 040;
+    YMSB++;
   if (byte & 1)
-    YA &= 03740;
+    YLSB = 0;
 }
 
 static void
@@ -382,7 +403,7 @@ dp_deim (uint16 insn)
 static void
 dp_dlvh (uint16 insn1, uint16 insn2, uint16 insn3)
 {
-  uint16 x1 = XA, y1 = YA;
+  uint16 x1 = deflect (XMSB, XLSB), y1 = deflect (YMSB, YLSB);
   uint16 m, n, dx, dy;
   m = insn2 & 07777;
   n = insn3 & 07777;
@@ -394,15 +415,15 @@ dp_dlvh (uint16 insn1, uint16 insn2, uint16 insn3)
     dy = n;
   }
   if (insn3 & 040000)
-    XA -= SCALE * dx;
+    decrement (&XMSB, &XLSB, dx);
   else
-    XA += SCALE * dx;
+    increment (&XMSB, &XLSB, dx);
   if (insn3 & 020000)
-    YA -= SCALE * dy;
+    decrement (&YMSB, &YLSB, dy);
   else
-    YA += SCALE * dy;
+    increment (&YMSB, &YLSB, dy);
   if (insn2 & 020000)
-    crt_line (x1, y1, XA, YA);
+    crt_line (x1, y1, deflect (XMSB, XLSB), deflect (YMSB, YLSB));
 }
 
 static void
@@ -414,11 +435,13 @@ dp_insn (uint16 insn)
     break;
   case 1: /* DLXA */
     sim_debug (DBG, &dp_dev, "DLXA\n");
-    XA = (insn & 01777) << 1;
+    XMSB = (insn >> 4) & 0077;
+    XLSB = (insn << 1) & 0036;
     break;
   case 2: /* DLYA */
     sim_debug (DBG, &dp_dev, "DLYA\n");
-    YA = (insn & 01777) << 1;
+    YMSB = (insn >> 4) & 0077;
+    YLSB = (insn << 1) & 0036;
     break;
   case 3: /* DEIM */
     sim_debug (DBG, &dp_dev, "DEIM ");