From 7b65dc1657e7464351323b86a8fe07b96beea3cb Mon Sep 17 00:00:00 2001
From: Peter Evans <peter@modul8r.com>
Date: Wed, 21 Feb 2018 21:01:46 -0600
Subject: [PATCH] Add new BIM instruction (BIt imMediate mode)

This is not a real instruction in the 65c02 processor; I invented it for
the sole purpose of handling the specialized logic that is performed by
BIT in IMM mode. To be fair--I can imagine this really _was_ implemented
as a "separate" instruction on the chip! But I don't know that for sure.
---
 include/mos6502.enums.h |  1 +
 include/mos6502.h       |  1 +
 src/mos6502.bits.c      | 16 ++++++++++++++++
 src/mos6502.c           |  3 ++-
 src/mos6502.dis.c       |  1 +
 tests/mos6502.bits.c    | 17 +++++++++++++++++
 6 files changed, 38 insertions(+), 1 deletion(-)

diff --git a/include/mos6502.enums.h b/include/mos6502.enums.h
index d518c63..b756957 100644
--- a/include/mos6502.enums.h
+++ b/include/mos6502.enums.h
@@ -73,6 +73,7 @@ enum instruction {
     BCS,    // Branch on Carry Set
     BEQ,    // Branch on EQual to zero
     BIT,    // BIT test
+    BIM,    // BIt test (imMediate mode) (* not a real instruction in the processor; just used by us)
     BMI,    // Branch on MInus 
     BNE,    // Branch on Not Equal to zero
     BPL,    // Branch on PLus
diff --git a/include/mos6502.h b/include/mos6502.h
index 69d29f6..89cc6bf 100644
--- a/include/mos6502.h
+++ b/include/mos6502.h
@@ -182,6 +182,7 @@ DECL_INST(bcc);
 DECL_INST(bcs);
 DECL_INST(beq);
 DECL_INST(bit);
+DECL_INST(bim);
 DECL_INST(bmi);
 DECL_INST(bne);
 DECL_INST(bpl);
diff --git a/src/mos6502.bits.c b/src/mos6502.bits.c
index f9d693b..b4be238 100644
--- a/src/mos6502.bits.c
+++ b/src/mos6502.bits.c
@@ -74,6 +74,22 @@ DEFINE_INST(bit)
     }
 }
 
+/*
+ * The BIM instruction (which is made up--it's not a real instruction)
+ * is here to handle the specific use-case of a BIT instruction in
+ * immediate (IMM) mode. We do this in a separate instruction to avoid
+ * the need to add logic to the BIT instruction such that it has to know
+ * or care about its opcode or its address mode.
+ */
+DEFINE_INST(bim)
+{
+    // This is the same behavior as BIT
+    cpu->P &= ~MOS_ZERO;
+    if (!(cpu->A & oper)) {
+        cpu->P |= MOS_ZERO;
+    }
+}
+
 /*
  * Compute the bitwise-exclusive-or between the accumulator and operand,
  * and store the result in A.
diff --git a/src/mos6502.c b/src/mos6502.c
index f061491..5474221 100644
--- a/src/mos6502.c
+++ b/src/mos6502.c
@@ -35,7 +35,7 @@ static int instructions[] = {
     BVC, EOR, EOR, BAD, BAD, EOR, LSR, BAD, CLI, EOR, BAD, BAD, BAD, EOR, LSR, BAD, // 5x
     RTS, ADC, BAD, BAD, BAD, ADC, ROR, BAD, PLA, ADC, ROR, BAD, JMP, ADC, ROR, BAD, // 6x
     BVS, ADC, ADC, BAD, BAD, ADC, ROR, BAD, SEI, ADC, BAD, BAD, BAD, ADC, ROR, BAD, // 7x
-    BAD, STA, BAD, BAD, STY, STA, STX, BAD, DEY, BIT, TXA, BAD, STY, STA, STX, BAD, // 8x
+    BAD, STA, BAD, BAD, STY, STA, STX, BAD, DEY, BIM, TXA, BAD, STY, STA, STX, BAD, // 8x
     BCC, STA, STA, BAD, STY, STA, STX, BAD, TYA, STA, TXS, BAD, BAD, STA, BAD, BAD, // 9x
     LDY, LDA, LDX, BAD, LDY, LDA, LDX, BAD, TAY, LDA, TAX, BAD, LDY, LDA, LDX, BAD, // Ax
     BCS, LDA, LDA, BAD, LDY, LDA, LDX, BAD, CLV, LDA, TSX, BAD, LDY, LDA, LDX, BAD, // Bx
@@ -65,6 +65,7 @@ static mos6502_instruction_handler instruction_handlers[] = {
     INST_HANDLER(bcs),
     INST_HANDLER(beq),
     INST_HANDLER(bit),
+    INST_HANDLER(bim),
     INST_HANDLER(bmi),
     INST_HANDLER(bne),
     INST_HANDLER(bpl),
diff --git a/src/mos6502.dis.c b/src/mos6502.dis.c
index 78b32ed..8921fa0 100644
--- a/src/mos6502.dis.c
+++ b/src/mos6502.dis.c
@@ -49,6 +49,7 @@ static char *instruction_strings[] = {
     "BCS",
     "BEQ",
     "BIT",
+    "BIM",
     "BMI",
     "BNE",
     "BPL",
diff --git a/tests/mos6502.bits.c b/tests/mos6502.bits.c
index 88738c8..93213b8 100644
--- a/tests/mos6502.bits.c
+++ b/tests/mos6502.bits.c
@@ -56,6 +56,23 @@ Test(mos6502_bits, bit)
     cr_assert_eq(cpu->P & MOS_ZERO, MOS_ZERO);
 }
 
+Test(mos6502_bits, bim)
+{
+    // This version of BIT should not modify the NV flags
+    cpu->P |= MOS_NEGATIVE;
+    cpu->P |= MOS_OVERFLOW;
+
+    cpu->A = 63;
+    mos6502_handle_bim(cpu, 123);
+    cr_assert_eq(cpu->P & MOS_ZERO, 0);
+    cr_assert_eq(cpu->P & MOS_NEGATIVE, MOS_NEGATIVE);
+    cr_assert_eq(cpu->P & MOS_OVERFLOW, MOS_OVERFLOW);
+
+    cpu->A = 4;
+    mos6502_handle_bim(cpu, 123);
+    cr_assert_eq(cpu->P & MOS_ZERO, MOS_ZERO);
+}
+
 Test(mos6502_bits, eor)
 {
     cpu->A = 5;