4190

1 files changed, 17 insertions, 26 deletions

diff --git a/subx/014index_addressing.cc b/subx/014index_addressing.cc
index 723a243a..f457c761 100644
--- a/subx/014index_addressing.cc
+++ b/subx/014index_addressing.cc
@@ -9,7 +9,8 @@
 # ModR/M in binary: 00 (indirect mode) 011 (src EBX) 100 (dest in SIB)
 # SIB in binary: 00 (scale 1) 100 (no index) 000 (base EAX)
 +run: add EBX to r/m32
-+run: effective address is 0x60 (EAX)
++run: effective address is initially 0x60 (EAX)
++run: effective address is 0x60
 +run: storing 0x00000011
 
 :(before "End Mod 0 Special-cases")
@@ -22,38 +23,27 @@ case 4:  // exception: mod 0b00 rm 0b100 => incoming SIB (scale-index-base) byte
 uint32_t effective_address_from_sib(uint8_t mod) {
   uint8_t sib = next();
   uint8_t base = sib&0x7;
-  if (base == EBP) {
-    // Need to sometimes use a displacement either in addition to or in place
-    // of EBP. This gets complicated, and I don't understand interactions with
-    // displacement mode in Mod/RM. For example:
-    //
-    // op (hex)   ModR/M (binary)                                  SIB (binary)                                      displacement (hex)
-    // 0x01       01 /*indirect+disp8*/ 000 /*EAX*/ 100 /*SIB*/    00 /*scale*/ 100 /*no index*/ 101 /*EBP+disp8*/   0xf0
-    //
-    // Do the two displacements accumulate (so the instruction has *two*
-    // displacement fields)?
-    //
-    // Maybe they're redundant:
-    //   "When the ModR/M or SIB tables state that a disp value is required..
-    //   then the displacement bytes are required."
-    //   -- https://wiki.osdev.org/X86-64_Instruction_Encoding#Displacement
-    //
-    // That's the only option that makes sense for 32-bit displacement (mod 10)
-    raise << "base 5 (often but not always EBP) not supported in SIB byte\n" << end();
-    return 0;
+  uint32_t addr = 0;
+  if (base != EBP || mod != 0) {
+    addr = Reg[base].u;
+    trace(2, "run") << "effective address is initially 0x" << std::hex << addr << " (" << rname(base) << ")" << end();
+  }
+  else {
+    // base == EBP && mod == 0
+    addr = imm32();
+    trace(2, "run") << "effective address is initially 0x" << std::hex << addr << " (disp32)" << end();
   }
   uint8_t index = (sib>>3)&0x7;
   if (index == ESP) {
     // ignore index and scale
-    trace(2, "run") << "effective address is 0x" << std::hex << Reg[base].u << " (" << rname(base) << ")" << end();
-    return Reg[base].u;
+    trace(2, "run") << "effective address is 0x" << std::hex << addr << end();
   }
   else {
     uint8_t scale = (1 << (sib>>6));
-    uint32_t addr = Reg[base].u + Reg[index].i*scale;  // treat index register as signed. Maybe base as well? But we'll always ensure it's non-negative.
-    trace(2, "run") << "effective address is 0x" << std::hex << addr << " (" << rname(base) << " + " << rname(index) << "*" << NUM(scale) << ")" << end();
-    return addr;
+    addr += Reg[index].i*scale;  // treat index register as signed. Maybe base as well? But we'll always ensure it's non-negative.
+    trace(2, "run") << "effective address is 0x" << std::hex << addr << " (after adding " << rname(index) << "*" << NUM(scale) << ")" << end();
   }
+  return addr;
 }
 
 :(scenario add_r32_to_mem_at_base_r32_index_r32)
@@ -66,5 +56,6 @@ uint32_t effective_address_from_sib(uint8_t mod) {
 # ModR/M in binary: 00 (indirect mode) 011 (src EBX) 100 (dest in SIB)
 # SIB in binary: 00 (scale 1) 001 (index ECX) 000 (base EAX)
 +run: add EBX to r/m32
-+run: effective address is 0x60 (EAX + ECX*1)
++run: effective address is initially 0x5e (EAX)
++run: effective address is 0x60 (after adding ECX*1)
 +run: storing 0x00000011