x16BitEmulation

This repository is basically me trying to emulate 6502 processor which is 16 bit fully on software level at a logical standpoint. This doesn't support any kind of graphical interface.

Disclaimer

This main goal of this project is to understand how CPUs works by directly emulating one and to debug it by single stepping instructions. The code is meant to be readable and understandable without many comments but more for writing very cohesive testings using Gtest, a lot of things could be done better. The emulator only shows you what's going on under the hood of a 6502 CPU, without displaying stuff graphically.

This was built on Visual Studio 2019 but has been upgraded to Visual Studio 2022. Retargeting the solution shouldn't cause any issues because of very less dependencies (only nuget package for Google test is required which should be automatically installed).

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Currently supported intructions

Load/Store Operations

These instructions transfer a single byte between memory and one of the registers. Load operations set the negative (N) and zero (Z) flags depending on the value of transferred. Store operations do not affect the flag settings.

Done	Code	Full Form	Flags Changed
Yes	LDA	Load Accumulator	N,Z
Yes	LDX	Load X Register	N,Z
Yes	LDY	Load Y Register	N,Z
Yes	STA	Store Accumulator	None
Yes	STX	Store X Register	None
Yes	STY	Store Y Register	None

Jumps & Calls

The following instructions modify the program counter causing a break to normal sequential execution. The JSR instruction pushes the old PC onto the stack before changing it to the new location allowing a subsequent RTS to return execution to the instruction after the call.

Done	Code	Second Header	Flags Changed
Yes	JMP	Jump to another location	None
Yes	JSR	Jump to a subroutine	None
Yes	RTS	Return from subroutine	none

Register Transfers

The contents of the X and Y registers can be moved to or from the accumulator, setting the negative (N) and zero (Z) flags as appropriate.

Done	Code	Second Header	Flags Changed
Yes	TAX	Transfer accumulator to X	N,Z
Yes	TAY	Transfer accumulator to Y	N,Z
Yes	TXA	Transfer X to accumulator	N,Z
Yes	TYA	Transfer Y to accumulator	N,Z

Stack Operator

The 6502 microprocessor supports a 256 byte stack fixed between memory locations $0100 and $01FF. A special 8-bit register, S, is used to keep track of the next free byte of stack space. Pushing a byte on to the stack causes the value to be stored at the current free location (e.g. $0100,S) and then the stack pointer is post decremented. Pull operations reverse this procedure.

The stack register can only be accessed by transferring its value to or from the X register. Its value is automatically modified by push/pull instructions, subroutine calls and returns, interrupts and returns from interrupts.

Done	Code	Second Header	Flags Changed
Yes	TSX	Transfer stack pointer to X	N,Z
Yes	TXS	Transfer X to stack pointer	None
Yes	PHA	Push accumulator on stack	None
Yes	PHP	Push processor status on stack	None
Yes	PLA	Pull accumulator from stack	N,Z
Yes	PLP	Pull processor status from stack	All

Logical

The following instructions perform logical operations on the contents of the accumulator and another value held in memory. The BIT instruction performs a logical AND to test the presence of bits in the memory value to set the flags but does not keep the result.

Done	Code	Second Header	Flags Changed
Yes	AND	Logical AND	N,Z
Yes	EOR	Exclusive OR	N,Z
Yes	ORA	Logical Inclusive OR	N,Z
Yes	BIT	Bit Test	N,V,Z

Arithmetic

The arithmetic operations perform addition and subtraction on the contents of the accumulator. The compare operations allow the comparison of the accumulator and X or Y with memory values.

Done	Code	Second Header	Flags Changed
Yes	ADC	Add with Carry	N,V,Z,C
Yes	SBC	Subtract with Carry	N,V,Z,C
Yes	CMP	Compare accumulator	N,Z,C
Yes	CPX	Compare X register	N,Z,C
Yes	CPY	Compare Y register	N,Z,C

Increments & Decrements

Increment or decrement a memory location or one of the X or Y registers by one setting the negative (N) and zero (Z) flags as appropriate,

Done	Code	Second Header	Flags Changed
Yes	INC	Increment a memory location	N,Z
Yes	INX	Increment the X register	N,Z
Yes	INY	Increment the Y register	N,Z
Yes	DEC	Decrement a memory location	N,Z
Yes	DEX	Decrement the X register	N,Z
Yes	DEY	Decrement the Y register	N,Z

Shifts

Shift instructions cause the bits within either a memory location or the accumulator to be shifted by one bit position. The rotate instructions use the contents if the carry flag (C) to fill the vacant position generated by the shift and to catch the overflowing bit. The arithmetic and logical shifts shift in an appropriate 0 or 1 bit as appropriate but catch the overflow bit in the carry flag (C).

Done	Code	Second Header	Flags Changed
Yes	ASL	Arithmetic Shift Left	N,Z,C
Yes	LSR	Logical Shift Right	N,Z,C
Yes	ROL	Rotate Left	N,Z,C
Yes	ROR	Rotate Right	N,Z,C

Branches

Branch instructions break the normal sequential flow of execution by changing the program counter if a specified condition is met. All the conditions are based on examining a single bit within the processor status.

Done	Code	Second Header	Flags Changed
Yes	BCC	Branch if carry flag clear	None
Yes	BCS	Branch if carry flag set	None
Yes	BEQ	Branch if zero flag set	None
Yes	BMI	Branch if negative flag set	None
Yes	BNE	Branch if zero flag clear	None
Yes	BPL	Branch if negative flag clear	None
Yes	BVC	Branch if overflow flag clear	None

Branch instructions use relative address to identify the target instruction if they are executed. As relative addresses are stored using a signed 8 bit byte the target instruction must be within 126 bytes before the branch or 128 bytes after the branch.

Status Flag Changes

The following instructions change the values of specific status flags.

Done	Code	Second Header	Flags Changed
Yes	CLC	Clear carry flag	C
Yes	CLD	Clear decimal mode flag	D
Yes	CLI	Clear interrupt disable flag	I
Yes	CLV	Clear overflow flag	V
Yes	SEC	Set carry flag	C
Yes	SED	Set decimal mode flag	D
Yes	SEI	Set interrupt disable flag	I

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Currently the architecture and spec sheet can be found here https://cx16.dk/6502/