BIT: Difference between revisions

Latest revision as of 04:50, 6 December 2023


Basic Info
Addressing Mode	Opcode	Length	Speed
Immediate	89	2 bytes*	2 cycles*
Absolute	2C	3 bytes	4 cycles*
Direct Page	24	2 bytes	3 cycles*
absolute indexed X	3C	3 bytes	4 cycles*
direct page indexed X	34	2 bytes	4 cycles*


Flags Affected
Addressing Mode	N	V	M	X	D	I	Z	C
Immediate	.	.	.	.	.	.		.
other			.	.	.	.		.

BIT is a 65x instruction that performs a logical AND operation between the accumulator and memory without storing the conjunction. If the conjunction is zero, the zero flag is set, otherwise it is cleared.

Except in immediate addressing, the most significant bit of the data located at the effective address is moved into the negative flag, and the second most significant bit of that data is moved into the overflow flag. BIT is often used right before a conditional branch instruction.

Cycle Penalties

BIT takes an extra cycle when the accumulator is 16 bits wide, in all addressing modes
In direct page addressing modes, BIT takes an extra cycle when the low byte of the direct page register is nonzero
In Absolute Indexed, X Addressing, BIT takes an extra cycle when adding the index crosses a page boundary

External Links

Eyes & Lichty page 431, on BIT: https://archive.org/details/0893037893ProgrammingThe65816/page/n457
Labiak page 121 on BIT: https://archive.org/details/Programming_the_65816/page/n131
MCS6500 Manual page 47 on BIT: https://archive.org/details/mos_microcomputers_programming_manual/page/n62
Carr page 249 on BIT: https://archive.org/details/6502UsersManual/page/n262
Leventhal page 3-45 on BIT: https://archive.org/details/6502-assembly-language-programming/page/n94
snes9x implementation of BIT: https://github.com/snes9xgit/snes9x/blob/master/cpuops.cpp#L265
undisbeliever on BIT: https://undisbeliever.net/snesdev/65816-opcodes.html#bit-test-memory-bits-against-accumulator

@@ Line 1: / Line 1: @@
-'''BIT''' is a 65x instruction that performs a logical AND operation between the [[accumulator]] and memory without storing the conjunction.
+{| class="wikitable" style="float:right;clear:right;width:40%"
-{| class="wikitable" style="float:right;clear:right;width:30%"
 !colspan="8"|Basic Info
 |+
@@ Line 9: / Line 7: @@
 |'''Speed'''
 |+
-|immediate
+|[[Immediate]]
 |89
-|2 bytes
+|2 bytes*
-|2 cycles
+|2 cycles*
 |+
-|absolute
+|[[Absolute]]
 |2C
 |3 bytes
-|4 cycles
+|4 cycles*
 |+
-|direct page
+|[[Direct Page Addressing|Direct Page]]
 |24
 |2 bytes
-|3 cycles
+|3 cycles*
 |+
 |absolute indexed X
 |3C
 |3 bytes
-|4 cycles
+|4 cycles*
 |+
 |direct page indexed X
 |34
 |2 bytes
-|4 cycles
+|4 cycles*
 |}
 {| class="wikitable" style="float:right;clear:right;width:30%"
-!colspan="9"|Flags Clobbered
+!colspan="9"|Flags Affected
+|+
+|Addressing Mode
+|[[Negative Flag|N]]
+|[[Overflow Flag|V]]
+|[[M Flag|M]]
+|[[X Flag|X]]
+|[[Decimal Flag|D]]
+|[[I Flag|I]]
+|[[Zero Flag|Z]]
+|[[Carry Flag|C]]
 |+
-|N
+|[[Immediate]]
-|V
+|.
-|M
+|.
-|X
+|.
-|D
+|.
-|I
+|.
-|Z
+|.
-|C
+|
+|.
 |+
+|other
 |
 |
@@ Line 56: / Line 66: @@
 |.
 |}
+'''BIT''' is a 65x instruction that performs a logical AND operation between the [[accumulator]] and memory without storing the conjunction.  If the conjunction is zero, the [[zero flag]] is set, otherwise it is cleared.
+Except in [[immediate addressing]], the most significant bit of the data located at the effective address is moved into the [[negative flag]], and the second most significant bit of that data is moved into the [[overflow flag]].  BIT is often used right before a conditional branch instruction.
+===== Cycle Penalties =====
+* BIT takes an extra cycle when the accumulator is 16 bits wide, in all [[addressing modes]]
+* In direct page addressing modes, BIT takes an extra cycle when the low byte of the [[direct page register]] is nonzero
+* In [[Absolute Indexed, X Addressing]], BIT takes an extra cycle when adding the index crosses a page boundary
+=== See Also ===
+* [[AND]]
+* [[BPL]]
+* [[BNE]]
+* [[BCC]]
+* [[BCS]]
+* [[BVC]]
+* [[BVS]]
 === External Links ===
-* [[Eyes & Lichty]] page on BIT: https://archive.org/details/0893037893ProgrammingThe65816/page/n457
+* [[Eyes & Lichty]] page 431, on BIT: https://archive.org/details/0893037893ProgrammingThe65816/page/n457
-* [[Labiak]] page on BIT: https://archive.org/details/Programming_the_65816/page/n131
+* [[Labiak]] page 121 on BIT: https://archive.org/details/Programming_the_65816/page/n131
-* [[MCS6500 Manual]] page on BIT: https://archive.org/details/mos_microcomputers_programming_manual/page/n62
+* [[MCS6500 Manual]] page 47 on BIT: https://archive.org/details/mos_microcomputers_programming_manual/page/n62
-* [[Carr]] page on BIT: https://archive.org/details/6502UsersManual/page/n262
+* [[Carr]] page 249 on BIT: https://archive.org/details/6502UsersManual/page/n262
-* [[Leventhal]] page on BIT: https://archive.org/details/6502-assembly-language-programming/page/n94
+* [[Leventhal]] page 3-45 on BIT: https://archive.org/details/6502-assembly-language-programming/page/n94
 * snes9x implementation of BIT: https://github.com/snes9xgit/snes9x/blob/master/cpuops.cpp#L265
 * undisbeliever on BIT: https://undisbeliever.net/snesdev/65816-opcodes.html#bit-test-memory-bits-against-accumulator