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

@@ -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

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);

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.

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),

src/mos6502.dis.c

@@ -49,6 +49,7 @@ static char *instruction_strings[] = {
     "BCS",
     "BEQ",
     "BIT",
+    "BIM",
     "BMI",
     "BNE",
     "BPL",

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;