6502 Comparison operations

A comparison is simply a subtraction that doesn’t save the results but sets the status flags. It doesn’t set the overflow flag however so implementation is a bit harder than it should be. In anticipation of this instruction I wrote the subtract function to return the result instead of saving it so I could just use the subtraction function. Sadly I noticed the missing flag just when I was getting ready to implement this function so ended up writing a new version. This is probably for the best as the subtraction routine would add a lot of inefficiency to the process so this will work a lot faster.

    private fun performCompare(first:Int, second:Int) {

        val sub = first - second

        setNumberFlags(sub)

        adjustFlag(CARRY_FLAG, (state.flags and NEGATIVE_FLAG) == 0 )

    }

The one reason why you might want to use the proper subtraction method would be in the case of binary coded decimal (BCD) numbers, but when you think about it for a minute it becomes obvious that the flag results would be the same for BCD numbers the only thing that would be different would be the result of the subtraction and as that is discarded anyway this much faster function is ideal for our needs.

The CMP instruction compares the contents of the accumulator with the indicated memory by subtracting the memory from the accumulator while leaving the contents of the accumulator alone. This means that if the accumulator is equal to the memory the zero flag will be set. In cases where the accumulator is greater than the memory the carry flag will be set while the negative flag will be clear. In cases where the accumulator is less than the memory then the carry flag will be clear while the negative flag will be set.

CMP ValueToCompare

BMI lessThan

BEQ equals

; code for greater than here

There are also versions of compare for the x register and the y register, which are CPX and CPY. These work the same as CMP but using their respective register instead of the accumulator. This is good as it allows more control over looping, for instance, here is a routine for copying 20 bytes from Source to Dest.

LDX #0

CopyLoop:

LDA Source,X

STA Dest,X

CPX #20

BMI CopyLoop

CMP – CoMPare accumulator with memory

Address Mode	Decimal OPCode	Hexadecimal OpCode	Size	Cycles
#Immediate	201	$C9	2	2
Zero Page	197	$C5	2	3
Zero Page,X	213	$D5	2	4
Absolute	205	$CD	3	4
Absolute,X	221	$DD	3	4-5
Absolute,Y	217	$D9	3	4-5
(Indirect, X)	193	$C1	2	6
(Indirect),Y	209	$D1	2	5-6

Flags affected: CNZ

Cycle Notes: Indirect and indexed modes take extra cycle if page boundaries crossed

Usage: Compare accumulator with memory setting flags as if memory was subtracted from accumulator. Z set if equals. C set if accumulator greater than or equal to memory. N set if accumulator is less than memory.

Test Code:

; CMP Immediate

  CLD

  LDA #0

  TAX

loop: INX

  CLC

  ADC #2

  CMP #20

  BNE loop

  BRK

  ; expect x=10,a=20,z=1

 

 

; CMP Zero Page

  CLD

  LDA #0

  TAX

loop: INX

  CLC

  ADC #2

  CMP 200

  BMI loop

  BRK

.ORG 200

.BYTE 20

  ; expect x=10,a=20,z=1

 

; CMP Zero Page,X

  CLD

  LDA #0

  TAY

  LDX #10

loop: INY

  CLC

  ADC #2

  CMP 190,X

  BCC loop

  BRK

.ORG 200

.BYTE 20

  ; expect x=10,a=20,z=1

; CMP Absolute

  CLD

  LDA #0

  TAX

loop: INX

  CLC

  ADC #2

  CMP 512

  BMI loop

  BRK

.ORG 512

.BYTE 20

  ; expect x=10,a=20,z=1

 

; CMP Absolute,X find 4 index

  LDA #4

  LDX #255

loop: INX

  CMP 512,X

  BNE loop

  BRK

.ORG 512

.BYTE 1 2 3 4

  ; expect A=4, X=3, Z=1, N=0, C=1

; CMP Absolute,Y

  LDA #5

  LDY #255

loop: INY

  CMP 512,Y

  BNE loop

  BRK

.ORG 512

.BYTE 1 2 3 4 5

  ; expect A=5, X=4, Z=1, N=0, C=1

; CMP (Indirect,X)

  LDA #69

  LDX #2

  LDY #0

  CMP (200,X)

  BEQ done

  LDY 512

done: BRK 

.ORG 200

.WORD 0 512

.ORG 512

.BYTE 42

  ; Expect A=69,Y=42

 

; CMP (Indirect),Y

  LDA #3

  LDY #255

loop: INY

  CMP (200),Y

  BNE loop

  BRK

.ORG 200

.WORD 512

.ORG 512

.BYTE 1 2 3 4 5

  ; expect A=3, X=2, Z=1, N=0, C=1

Implementation:

// #immediate

performCompare(state.acc, mem.read(state.ip+1))

// zero page

performCompare(state.acc, mem.read(mem.read(state.ip+1)))

// zero page,X

performCompare(state.acc, mem.read(mem.read(state.ip+1) + state.x))

//absolute

performCompare(state.acc, mem.read(findAbsoluteAddress(state.ip)))

//absolute,X

performCompare(state.acc, mem.read(findAbsoluteAddress(state.ip)+state.x))

//absolute,Y

performCompare(state.acc, mem.read(findAbsoluteAddress(state.ip)+state.y))

// (indirect, X)

performCompare(state.acc, mem.read(findAbsoluteAddress(((mem.read(state.ip+1)+state.x) and 255)-1)))

// (indirect),Y

performCompare(state.acc, mem.read(findAbsoluteAddress(mem.read(state.ip+1) -1) + state.y))

 CPX   Compare Memory and Index X

CPX – ComPare X register with memory

Address Mode	Decimal OPCode	Hexadecimal OpCode	Size	Cycles
#Immediate	224	$E0	2	2
Zero Page	228	$E4	2	3
Absolute	236	$EC	3	4

Flags affected: CNZ

Usage: Compare X Register with memory setting flags as if memory was subtracted from X. Z set if equals. C set if X greater than or equal to memory. N set if X is less than memory.

Test Code:

; CPX Immediate - compute 3x20 the hard way

  CLD

  LDA #0

  TAX

loop: INX

  CLC

  ADC #3

  CPX #20

  BNE loop

  BRK

  ; expect x=20,a=60,z=1

 

 

; CPX Zero Page - compute 5x5 the hard way

  CLD

  LDA #0

  TAX

loop: INX

  CLC

  ADC #5

  CPX 200

  BCC loop

  BRK

.ORG 200

.BYTE 5

  ; expect x=5,a=25,c=1

 

; CPX Absolute - Compute 6x7 the hard way

  CLD

  LDA #0

  TAX

loop: INX

  CLC

  ADC #7

  CPX 520

  BMI loop

  BRK

.ORG 520

.BYTE 6

  ; expect x=6,a=42,n=0

Implementation:

// #immediate

performCompare(state.x, mem.read(state.ip+1))

// zero page

performCompare(state.x, mem.read(mem.read(state.ip+1)))

//absolute

performCompare(state.x, mem.read(findAbsoluteAddress(state.ip)))

CPY – ComPare Y register with memory

Address Mode	Decimal OPCode	Hexadecimal OpCode	Size	Cycles
#Immediate	192	$C0	2	2
Zero Page	196	$C4	2	3
Absolute	204	$CC	3	4

Flags affected: CNZ

Usage: Compare Y Register with memory setting flags as if memory was subtracted from Y. Z set if equals. C set if Y greater than or equal to memory. N set if Y is less than memory.

Test Code:

; CPY Immediate - compute 3x20 the hard way

  CLD

  LDA #0

  TAY

loop: INY

  CLC

  ADC #3

  CPY #20

  BNE loop

  BRK

  ; expect y=20,a=60,z=1

 

 

; CPY Zero Page - compute 5x5 the hard way

  CLD

  LDA #0

  TAY

loop: INY

  CLC

  ADC #5

  CPY 200

  BCC loop

  BRK

.ORG 200

.BYTE 5

  ; expect Y=5,a=25,c=1

 

; CPY Absolute - Compute 6x7 the hard way

  CLD

  LDA #0

  TAY

loop: INY

  CLC

  ADC #7

  CPY 520

  BMI loop

  BRK

.ORG 520

.BYTE 6

  ; expect y=6,a=42,n=0

Implementation:

// #immediate

performCompare(state.y, mem.read(state.ip+1))

// zero page

performCompare(state.y, mem.read(mem.read(state.ip+1)))

//absolute

performCompare(state.y, mem.read(findAbsoluteAddress(state.ip)))

Math Branching

The last couple of post seen the ADC and the SBC instructions added to the 6502 emulator. We now have some instructions that cause the carry and overflow flags to be set. For multi-byte numbers the use of the carry flag is built into the ADC and SBC instructions and will be used automatically without need for special branching logic. The carry flag is necessary for dealing with adding or subtracting a byte from a multi-byte value. In this case you can write the code like this:

LDA NumberLowByte

CLC

ADC ByteToAdd

STA NumberLowByte

BCC noOverflow

INC NumberHighByte

noOverflow: ; additional code here

Probably the most important use for the carry flag branching  functions BCC amd BCS is for handling overflow errors when using unsigned numbers. When a number gets too big to be contained in the byte it sets the carry flag and wraps around. In cases where this is the only byte or the last byte in a multibyte number then there is a very good possibility that something bad has just happened. If this is a possibility in your code then you should be using BCC or BCS to handle potential errors.

Signed numbers overflow in a rather different way, which is why we have the overflow flag in addition to the carry flag. If you are using a signed byte then 64 + 64 results in -128 which is not what you want. The overflow flag solves this by being set when the sign of a number changes in cases where it shouldn’t be changing such as adding two positive numbers together or adding two negative numbers together.  The BVC and BVS commands are used to detect this problem and handle the error from this.

When dealing with multi-byte signed numbers, you need to be concerned with the overflow flag only for the highest byte of the number.

BCC – Branch on Carry Clear

Address Mode	Decimal OPCode	Hexadecimal OpCode	Size	Cycles
Relative	144	$90	2	2-4

Flags affected: None

Cycle Notes: If branching will take an additional cycle (3). If branch crosses page boundaries then an additional cycle will be added on top of the cycle for branching (4).

Usage: Checks to make sure last math operation has not resulted in a carry (ADC) or has borrowed (SBC). A carry is an indication that the bits in the byte have wrapped so this instruction will branch if the byte has not wrapped.

Test Code:

; BCC test by counting times can add 10 before wrapping

CLD

LDA #0

TAX

count: INX

CLC

ADC #10

BCC count

BRK

; expect A = 6, X=26, C=1

Implementation:

processBranch(state.flags and CARRY_FLAG != CARRY_FLAG, mem.read(state.ip+1) )

BCS – Branch on Carry Set

Address Mode	Decimal OPCode	Hexadecimal OpCode	Size	Cycles
Relative	176	$B0	2	2-4

Flags affected: None

Cycle Notes: If branching will take an additional cycle (3). If branch crosses page boundaries then an additional cycle will be added on top of the cycle for branching (4).

Usage: Checks to make sure last math operation has not resulted in a carry (ADC) or has borrowed (SBC). A carry is an indication that the bits in the byte have wrapped so with this command the branch will be taken if the byte has wrapped.

Test Code:

; BCS count how many times can subtract 10 before wrapping

CLD

LDA #255

LDX #0

count: INX

SEC

SBC #10

BCS count

BRK;

; expect A 251, X26, C=0

Implementation:

processBranch(state.flags and CARRY_FLAG == CARRY_FLAG, mem.read(state.ip+1) )

BVC   Branch on Overflow Clear

BVC – Branch on oVerflow Clear

Address Mode	Decimal OPCode	Hexadecimal OpCode	Size	Cycles
Relative	80	$50	2	2-4

Flags affected: None

Cycle Notes: If branching will take an additional cycle (3). If branch crosses page boundaries then an additional cycle will be added on top of the cycle for branching (4).

Usage: Checks to make sure the last math operation didn’t cause an overflow. This is for signed numbers when a number changes its sign when it shouldn’t due to too large of numbers being used.

Test Code:

; BVC count how many times can add 10 until become negative

CLD

LDA #0

TAX

count: INX

CLC

ADC #10

BVC count

BRK;

; expect A 130, X13, C=0, V=1

Implementation:

processBranch(state.flags and OVERFLOW_FLAG != OVERFLOW_FLAG, mem.read(state.ip+1) )

BVS   Branch on Overflow Set

BVS – Branch on oVerflow Set

Address Mode	Decimal OPCode	Hexadecimal OpCode	Size	Cycles
Relative	112	$70	2	2-4

Flags affected: None

Cycle Notes: If branching will take an additional cycle (3). If branch crosses page boundaries then an additional cycle will be added on top of the cycle for branching (4).

Usage: Checks if last math operation caused an overflow. This is for signed numbers when a number changes its sign when it shouldn’t due to too large of numbers being used.

Test Code:

; BVS test

CLD

LDA #64

LDX #0

CLC

ADC #32

BVS done

INX

CLC

ADC #32

BVS done

INX

done: BRK

; expect A=128, X=1, V=1

Implementation:

processBranch(state.flags and OVERFLOW_FLAG == OVERFLOW_FLAG, mem.read(state.ip+1) )