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#6502 EMULATOR C FULL#
#6502 EMULATOR C SOFTWARE#
#6502 EMULATOR C CODE#

The folks at WDC recently contacted me to see if I would give their hardware a close look, and after providing a few boards, this hardware proved to be both excellent. Recently, WDC has been interested in the educational aspects of the 6502, with one of the VPs,, lending his time to an after-school club teaching opcodes. For more than 35 years, WDC has been the home of 6502-related designs. The cathedral of The 6502 is Western Design Center. Combine the longevity of the 6502 with the fact an entire generation of developers first cut their teeth on 6502 assembly, and you have the makings of a classic microprocessor that will, I’m sure, still be relevant in another forty years. The history of the 6502 doesn’t end with video games for the last forty years, this CPU has found its way into industrial equipment, medical devices, and everything else that doesn’t need to be redesigned every two years. Since 6502 systems are no longer very common, a web-based 6502 Emulator is available for assembling, testing, and debugging 6502 Assembly code.The 6502 is a classic piece of computing history. Versions of this CPU were found in everything from the Apple ][, to the Nintendo Entertainment System, and the Commodore 64.

#6502 EMULATOR C FULL#

See the references (below) for the full details of the 6502 instruction set. LDA $f005 load the accumulator with the contents of memory location $f005 LDA $05 load the accumulator with the contents of memory location 5 (in the zero page) LDA #$05 load the accumulator with the number 5 6502 Assembly Language uses 3-letter menomics to specify the operation, and argument syntax to specify the addressing mode. Each opcode corresponded to an instruction, which consists of an operation and an addressing mode. The 6502 instruction set consist of a number of single-byte opcodes, each of which is followed by 0, 1, or 2 bytes of arguments.

S - Negative Sign - set when an operation produces a negative result.

V - Overflow - Set when a math operation overflows or underflows a one-byte signed result.

#6502 EMULATOR C SOFTWARE#

B - Break - Indicates a software interrupt rather than a hardware interrupt has occurred.

D - Decimal mode - bytes are interpreted as two-digit decimal values instead of 8-bit binary values when doing math.

Z - Zero flag - indicates that an operation produced a zero or equal result.

C - Carry - Used to indicate a carry or borrow during addition/subtraction.

Processor Status (P or PS) - a collection of bits (flags) which indicate or control aspects of the processor mode and status:.

Stack Pointer (S or SP) - a pointer to the current position in the stack.

Program Counter (PC) - a pointer to the currently-executing instruction in memory.

There are also three special-purpose registers: Some index operations may only be performed with a specific index register (X or Y, but not interchangeably).

Y Index (Y) - a register similar to the X register.

X Index (X) - a register which can be used for limited math operations as well as indexed addressing modes, where an index value is added to a base address for memory operations.

Accumulator (A) - the main register for math operations.

There are three general-purpose registers: In addition, each system built using the 6502 would have hardware devices, such as the video system, keyboard, and communication interfaces, occupying a portion of the address space.

#6502 EMULATOR C CODE#

$FE contains a pointer to code which is run when an interrupt request is received $FC contains a pointer to code which is run when the CPU is reset (including when it is first started) and $FA contains a pointer to code which is run when a non-maskable interrupt (NMI) is received. The last 6 bytes of this page contain three 2-byte addresses.

The stack wraps around, so if more than 256 bytes are pushed, the oldest values will be overwritten. The stack descends as it is used - more recently-pushed values are stored at lower addresses than older values. Values are pushed to, and pulled (popped) from, this region in first-in last-out (FIFO) order. Some pages have special, pre-defined purposes: Thus, a pointer at memory location $0010, which points to memory location $ABCD, would be stored like this: Since each 16-bit address is comprised of two 8-bit bytes, memory can be viewed as 256 pages of 256 bytes each.Įach pointer in memory is stored in two consecutive memory locations, with the lowest-value byte stored first this is known as Little Endian order. It is therefore able to access 64 kilobytes (2 16 bytes). The 6502 is an 8-bit processor with a 16-bit address bus.