If the branch is taken, the new address will the the current PC plus the offset. The byte after the opcode is the branch offset. Relative addressing on the 6502 is only used for branch operations. For example, the CLC instruction is implied, it is going to clear the processor's Carry flag. In an implied instruction, the data and/or destination is mandatory for the instruction. If $B2 was written without a #, it would indicate an address or offset. Remember that in assembly when you see a # sign, it indicates an immediate value. ORA #$B2 will perform a logical (also called bitwise) of the value B2 with the accumulator. These instructions have their data defined as the next byte after the opcode. These instructions have register A (the accumulator) as the target. Most of the addressing modes are used to access memory, but 3 do not. That seems to be a logical way to discuss them, so I'll stick with that. When the 6502 refers to addressing modes, it really means "What is the source of the data used in this instruction?" The 6502's data book divides the addressing modes into 2 groups, indexed and non-indexed. An interactive disassembler emulating a machine environment is to single stepping what poetry is to tax forms.Emulator 101 8080 reference 6502 reference 6502 Addressing Modes
I'm not sure how to explain well, but we don't learn about say driving cars by looking at pictures of people driving cars, we just go experience driving.
Implementing the ability to emulate the chip at various clock rates and running software built for specific purpose will be more useful for understanding software and hardware, rather than say providing opcodes and operands and seeing state changes in 'snapshots'. Eg if you wrote an arcade game emulator for a given cabinet but didn't get the timing right, in your processor implementation or in the specifics of how it was supposed to be interacting with other hardware (as mentioned, via irq pins, etc), if you inspected the memory associated with the game's framebuffer you wouldn't find a rendered game frame there you might find nonsense, or the program might enter a never-ending loop, who knows, but it would not be likely to be a correct result.įor the purposes of being a learning tool and so forth, it's likely important to build a few emulations of things that made use of a 6502, because software interacts with the environment through the cpu, and the environment interacts with software through the cpu. So if the lines weren't being set low at some rate corresponding to "the screen is such and such amount rendered", the program would not produce correct output. Eg in an old arcade machine you might have important tasks the program does tied to lines being set low by various conditions in the display (related to say, "the screen is now 3/4th rendered", etc). In other words the programs timing and how execution happens will change depending on what the program is and where it was running, because the environment is also changing the state of the processor, not just the program. If I have a program used in a chip in a phone or some such, the program will be designed in hand with the hardware, and will necessarily make use of say interrupt lines being set low under whatever conditions are relevant, and so forth.
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GLIBC, the GNU C Library documentation provides a manual (PDF, HTML), Wiki, and FAQ.The C Book second edition by Mike Banahan, Declan Brady and Mark Doran is freely available online.Written by the language author, and known colloquially as the "K & R" book-a book of lore The C Programming Language by Dennis M.Use this tool to format code as code FiltersĬlick the following link to filter out the chosen topic Only C is on topic (not C++, C# or general programming).Format your code properly (4 leading spaces, correctly indented).