From b63b3953d2420422f722a973d4093fafa320964f Mon Sep 17 00:00:00 2001
From: Mark Emmer <markemmer@SNOBOL4.com>
Date: Sat, 31 Jan 2015 23:30:17 -0600
Subject: [PATCH] SDS: Allow display or entry of NOP with optional operands

While debugging DDT subsystem of 940 time-sharing system, discovered NOP opcode used with indirection, non-zero operand address and tag field. I_V_OPO (opcode only) was redundant with I_V_NPN (no operand) opcode classes. Convert I_V_OPO to new "operand optional" class and use it for NOP, and add code to detect presence or absence of operand or tag for this class for machine code display and entry.
---
 SDS/sds_sys.c | 23 ++++++++++++++++-------
 1 file changed, 16 insertions(+), 7 deletions(-)

diff --git a/SDS/sds_sys.c b/SDS/sds_sys.c
index 07f65083..4cc33e34 100644
--- a/SDS/sds_sys.c
+++ b/SDS/sds_sys.c
@@ -294,7 +294,7 @@ return SCPE_NXM;
 #define I_V_MRF         003                             /* memory reference */
 #define I_V_REG         004                             /* register change */
 #define I_V_SHF         005                             /* shift */
-#define I_V_OPO         006                             /* opcode only */
+#define I_V_OPO         006                             /* operand optional */
 #define I_V_CHC         007                             /* chan cmd */
 #define I_V_CHT         010                             /* chan test */
 #define I_V_SPP         011                             /* system POP */
@@ -312,7 +312,7 @@ return SCPE_NXM;
 static const int32 masks[] = {
  037777777, 010000000, 017700000,                       /* NPN, PPO, IOI */
  017740000, 017700000, 017774000,                       /* MRF, REG, SHF */
- 017700000, 017377677, 027737677,                       /* OPO, CHC, CHT */
+ 017740000, 017377677, 027737677,                       /* OPO, CHC, CHT */
  057740000                                              /* SPP           */
  };
 
@@ -364,7 +364,7 @@ static const char *opcode[] = {                         /* Note: syspops must pr
          "BRU*",
  "MIY*", "BRI*", "MIW*", "POT*",
  "ETR*", "MRG*", "EOR*",
-         "EXU*",
+ "NOP*", "EXU*",
  "YIM*", "WIM*", "PIN*",
  "STA*", "STB*", "STX*",
          "BRX*", "BRM*",
@@ -437,7 +437,7 @@ static const int32 opc_val[] = {
                   000140000+I_MRF,                                       /*       BRU*,             */
  001040000+I_MRF, 001140000+I_MRF, 001240000+I_MRF, 001340000+I_MRF,     /* MIY*, BRI*, MIW*, POT*, */
  001440000+I_MRF, 001640000+I_MRF, 001740000+I_MRF,                      /* ETR*, MRG*, EOR*,       */
-                  002340000+I_MRF,                                       /*       EXU*,             */
+ 002040000+I_OPO, 002340000+I_MRF,                                       /* NOP*, EXU*,             */
  003040000+I_MRF, 003240000+I_MRF, 003340000+I_MRF,                      /* YIM*, WIM*, PIN*,       */
  003540000+I_MRF, 003640000+I_MRF, 003740000+I_MRF,                      /* STA*, STB*, STX*,       */
                   004140000+I_MRF, 004340000+I_MRF,                      /*       BRX*, BRM*,       */
@@ -560,8 +560,7 @@ for (i = 0; opc_val[i] >= 0; i++) {                     /* loop thru ops */
 
         switch (j) {                                    /* case on class */
 
-        case I_V_NPN:                                   /* no operands */
-        case I_V_OPO:                                   /* opcode only */
+        case I_V_NPN:                                   /* no operand */
             fprintf (of, "%s", opcode[i]);              /* opcode */
             break;
 
@@ -589,6 +588,13 @@ for (i = 0; opc_val[i] >= 0; i++) {                     /* loop thru ops */
                 fprintf (of, ",%-o", tag);
             break;
 
+        case I_V_OPO:                                   /* operand optional */
+            if (!tag && !va)
+            {
+                fprintf (of, "%s", opcode[i]);          /* opcode only */
+                break;
+            }                                           /* or fall through to MRF */
+
         case I_V_MRF:                                   /* mem ref */
             fprintf (of, "%s %-o", opcode[i], va);
             if (tag)
@@ -704,7 +710,7 @@ j = (opc_val[i] >> I_V_FL) & I_M_FL;                    /* get class */
 
 switch (j) {                                            /* case on class */
 
-    case I_V_NPN: case I_V_OPO:                         /* opcode only */
+    case I_V_NPN:                                       /* no operand */
         break;
 
     case I_V_SHF:                                       /* shift */
@@ -732,9 +738,12 @@ switch (j) {                                            /* case on class */
         val[0] = val[0] | d | tag;
         break;
 
+    case I_V_OPO:                                       /* operand optional */
     case I_V_SPP:                                       /* syspop */
     case I_V_MRF:                                       /* mem ref */
         cptr = get_glyph (cptr, gbuf, ',');             /* get next field */
+        if (gbuf[0]=='\0' && j==I_V_OPO)                /* operand optional */
+            break;
         d = get_uint (gbuf, 8, VA_MASK, &r);            /* virt address */
         if (r != SCPE_OK)
             return SCPE_ARG;