Questions tagged [6502]
For 6502 series of processors, including hardware and assembly language questions. Use with [hardware] for the hardware interface in particular.
140
questions
8
votes
0answers
119 views
Systems that used the Rockwell R65C00/21 or R65C29
The Rockwell R65C00/21 and R65C29 are described in an October 1984 datasheet ("R65C00/21 dual CMOS microcomputer and R65C29 dual CMOS microprocessor," Document no. 29651N64, rev. 2) which is ...
5
votes
1answer
314 views
Apple II Assembly DSECT
I'm trying to define a section of data in a program. The Apple 6502 Assembler/Editor mentions an assembler directive called "DSECT" but I can't tell if I'm using it correctly or not. Here is ...
24
votes
2answers
3k views
Was leaving all xxxxxx11 opcodes unused on the 6502 a deliberate design choice?
The 6502, like many 8-bit processors, has a somewhat arcane opcode-mode restrictions. On most such processors, the restriction is a clear result of trying to pack a lot of instructions into a limited ...
2
votes
0answers
101 views
How to properly convert an active-low joystick routine to active-high
My last question wasn't specific enough so I'll rephrase it. I've been having trouble with programming game consoles/computers like Commodore 64 that have active-low joysticks, where the bits are all ...
17
votes
3answers
3k views
What is the purpose of mirrored memory regions in NES's CPU memory map? [duplicate]
[Please see answers to this related question as well]
I've started reading the "official" NES Documentation and in page ten, it says that "memory locations $0000-$07FF are mirrored ...
6
votes
1answer
232 views
Commodore 64 sound seems to freeze at a high pitch if executed too soon after startup
I created this little sound driver for sound effects, it makes a classic "laser beam" sound you would hear in old arcade games. There's just one problem, and it's a bit of a strange one.
I ...
3
votes
1answer
132 views
Spontaneous movement and game crashes from active-low joystick routines [closed]
I've noticed this recurring problem when I'm trying to program joysticks or buttons that are active-low (0 = pressed), like on the Commodore 64 or Game Boy Advance. Regardless of the language or ...
4
votes
1answer
349 views
How to wait for vBlank on Commodore 64
I'm not new to 6502 Assembly but I am new to the Commodore 64. I'm used to the NES where you have to write everything yourself, such as the NMI/RESET/IRQ vectors, etc, but from what I understand the ...
5
votes
2answers
346 views
Macro for counted string accepting multiple arguments?
Using ca65 assembler, I'd like to use a macro to encode a string as a pascal style string or FORTH counted string with a leading length byte.
In ca65 documentation there is this example:
....
11
votes
3answers
2k views
Protecting 6502 decimal mode code from interrupts
A common idiom in 6502 assembly code is to protect sequences of code that rely on decimal mode from being interrupted. This because an interrupt handler typically clears decimal mode (cld), and thus ...
9
votes
3answers
665 views
C++ compilation for the Z80 and 6502?
I know it's almost heresy to ask something like this, but ...
We have C compilers for the Z80/6502 and there used to exist a thing called the Comeau C++ compiler, that produced C code as output. There ...
1
vote
1answer
71 views
Is there a way to use .ident or * (PC) in numeric macro variables (using .set)?
I'm trying to write a macro to create "headers" in a linked list fashion.
In each header the first 2 bytes should be the address of the previous header starting address (0000 in the first ...
6
votes
1answer
321 views
Where does the 6502 store the low order byte after fetching for read instructions?
I am trying to get a mental model of how the 6502 operates. I am going off this image:
However please suggest a better one if it helps understanding .
On this website it describes how a read ...
6
votes
1answer
587 views
Why does the JSR operation not jump?
I am building a computer based on the 65C02S, and when stepping through the code, I see strange things happening.
This is my code:
SEI
CLD
LDX #$ff
TXS
JSR someRoutine
...
This translates to this ...
6
votes
2answers
567 views
Why doesn't Visual6502 simulate RRA in the way I expect?
As everyone knows, the 6502 has many many undefined opcodes. One of these is RRA, which essentially performs both ROR and ADC.
From what I understand of hardware (really not much) and the explanation ...
14
votes
2answers
1k views
Why does the 6502 JSR instruction only increment the return address by 2 bytes?
Currently messing with 6502 assembly on a C64, and I don't understand why the JSR instruction is so weird.
According to the instruction table, JSR is a 3-byte instruction and only operates in absolute ...
2
votes
1answer
180 views
How to convert series of hex digits in memory into the format for use with Applesoft math subroutines
I'm trying to write a calculator program in assembly for the Apple //e. I know that there are built-in subroutines for performing all kinds of math operations involving the floating-point accumulator (...
8
votes
1answer
389 views
Why does the 6502 read from the stack before writing it?
I am building a W65C02S based computer for fun, and I try to build basically all the tools myself. Including the assembler (yes, I know they exist, I just want to make everything from scratch myself, ...
13
votes
1answer
2k views
How did the 6502 CPU get its name?
I am interested in the etymology of the name for the 6502 CPU. How were those numbers arrived at by the designers? Do they refer to anything specific? Is there some sort of origin story?
I found some ...
8
votes
1answer
842 views
6502 double store and copy protection?
I was watching a video on netlist simulation of the 6502. The presenter mentions how the 6502 has a double store upon using INCY, and then offhandedly mentions that this can be used for copy ...
6
votes
3answers
2k views
What is the MOS 6502 doing on each cycle of an instruction?
So, first off, I am kind of a noob with emulators and the 6502.
Summarise the problem
If we for example take the instruction ADC Immediate ($69) which adds the accumulator to an immediate value and ...
4
votes
2answers
446 views
How much memory does this SYM-1 have?
I have a SYM-1 with the following memory configuration:
How much memory is that? Those all appear to be L2114UCE chips, which are described as "Static 1024x4 NMOS RAM", but I'm not sure how ...
4
votes
1answer
545 views
Sym-1: What is the "f" register?
Running SUPERMON on a Synertek SYM-1, the R (REG) command cycles through a number of registers. Most are self explanatory:
p is the program counter (pc)
s is the stack pointer
a, x, and y are obvious
...
6
votes
1answer
180 views
Chaining IRQ's with BASIC & Kernal routines turned off
Having disabled BASIC and Kernal routines I set up an IRQ to run the following code when the raster line reaches 200:
.irq1
inc 53280 // change border colour
// lda #$ff // clear the ...
6
votes
4answers
1k views
Efficient way to JMP or JSR to an address stored somewhere else?
I'd like to JSR or JMP (preferably JMP as it's 3 cycles cheaper) to an address stored somewhere else in memory, the address of which is itself indexed.
I came up with the following self-modifying code ...
0
votes
1answer
235 views
Defining a C function with given address in cc65
I have a function that should be callable with the following syntax:
void __fastcall__ initSIDplayer1(uint8_t a);
The __fastcall__ modifier here means that the argument is passed in the Accu instead ...
0
votes
3answers
487 views
How was “Super Mario Bros” made with Assembly if the DASM compiler came out in 1987?
Would somebody care to explain how "Super Mario Bros." was released in 1985. But the DASM Assembler came out in 1987?
The 6502 Assembly language was introduced in 1975. How would you compile/...
4
votes
2answers
448 views
The use of segments in non-relocating assemblers
I've been looking at a bunch of different 6502 assemblers recently (dasm, ca65, xa, kickass), and many of them have support for multiple code segments. Some of these assemblers can generate ...
2
votes
1answer
97 views
Defining zeropage address labels in xa assembler?
I am trying to use the xa assembler to write some 6502 code. According to the documentation, I can define a zeropage segment using the .zero pseudo-op. I was hoping to do something like this:
.zero
...
7
votes
2answers
818 views
Basic / Assembler startup code (C64)
I'm just creating my own .prg file using NCurses Hexeditor in Ubuntu which I load using load"file.prg",8 in Vice. I followed the information in this link explaining how to create a .prg file....
10
votes
4answers
577 views
Commodore BASIC: why was the PET's GOTO so slow?
Received wisdom has it that Commodore didn't want to pay Microsoft extra for updated BASIC interpreters, and thus very little changed in Commodore BASIC v2 from the PETs to the C64. But consider the ...
3
votes
1answer
198 views
Is there a way to have the ca65 assembler only expand macros?
I would like to convert an assembly program in ca65 with macros into assembly code with resolved macros in order to carry out manual or automatic optimizations with the resulting code. Then, the ...
10
votes
4answers
1k views
Modulus arithmetic on the 6502
I have a very simple problem with is turning out to be surprisingly complex to solve.
I wish to add two bytes using modulus arithmetic, where the modulus is known at compile time. e.g. given modulus 5,...
8
votes
1answer
307 views
Is there a possibility for a user-defined IRQ hardware vector on a Commodore 128 in C128 mode?
When an IRQ occurs, the 6502 microprocessor and its derivatives in the C64 and C128 push the status register to the stack and then load the contents of a two-byte vector at memory address $FFFE/$FFFF ...
2
votes
1answer
286 views
Clear description of MS BASIC number → string conversions?
I'm failing to find a concise document that describes the formatting used by (6502) MS BASIC when converting internal floating-point numbers to text for display in PRINT, STR$, etc.
I know it has a ...
7
votes
2answers
556 views
Labels in cc65 inline assembler with #define macro
In general, it is possible to use inline assembler statements within C macros, for example
#define toscreencode(C) (__AX__ = (C), \
asm("cmp #$60"),\
...
9
votes
3answers
2k views
Why did decimal arithmetic slow down VisiCalc?
There is an excellent article about VisiCalc that goes into all the details about what happened and why, highly recommended if you are interested in that part of computing history. I was reading this ...
3
votes
2answers
332 views
Could the 6502 have worked with an Altair-style front panel?
I have fond memories of programming the 6502, though I never did any hardware hacking with it. I notice that the Altair, with its iconic front panel, was based on the Intel 8080, and from then on, ...
17
votes
1answer
1k views
Why did the 6502 handle BCD with a special mode?
The 6502 had special support for BCD arithmetic, because it was widely used in those days; this much, it had in common with other CPUs.
But the 8080 and 6800 implemented this in the form of a 'decimal ...
10
votes
5answers
3k views
Why were there no 32-bit or 64-bit versions of the m68k & 65xx lines of CPUs?
I don't understand why Western Design Center made the 65816 a 16-bit upgrade to the 6502 but Commodore Semiconductor Group/MOS Technology didn't make their own variant & why neither company made ...
0
votes
4answers
239 views
How did graphics processing for old 8/16bit consoles work & what are the differences between an FPU/maths co processor & a graphics card? [closed]
When looking on the internet at information about old consoles it's pretty easy to find out information on CPUs like the 6502, 65816, 68000 & Z80 used in retro computers & consoles like the ...
27
votes
6answers
7k views
Why are old CPUs like MOS Technology 6502 and Motorola 68000 considered better for real time systems applications than modern x86 based CPUs?
Reading the Wikipedia article about real-time computing, I found written that:
Once when the MOS Technology 6502 (used in the Commodore 64 and Apple II), and later when the Motorola 68000 (used in ...
3
votes
2answers
549 views
How were games and programs written for the C64 in assembler in the 80s? [duplicate]
Were developers using monitors like e.g. "Supermon+64", "Merlin 64" or "Monitor$C000" and cartridges like "Final Cartridge", "Action Replay" (both of ...
3
votes
1answer
288 views
When did the faster versions of the NMOS 6502 become commercially available?
The 6502 was released in 1975 with an advertised speed of about 1mhz but I can't find reliable information on what year faster versions became available (was 2mhz possible at launch?) or what the true ...
2
votes
1answer
308 views
Does the 6502's TXS and TSX affect flags or not?
Title says it all really. Some documentation claims that the S and Z flags are affected by both TXS and TSX, just as with all other transfer commands. Other documentation suggests that neither TXS nor ...
3
votes
2answers
576 views
Why don't all Absolute,X instructions take an extra cycle to cross page boundaries?
The question is about 6502 instructions that take an extra cycle when address calculation carries over into the high byte of the address. This affects addressing modes Absolute,X, Absolute,Y, Indirect,...
58
votes
8answers
14k views
Could you reverse engineer silicon just by looking at it?
I was interested in a recent interview of Masayuki Uemura, one of the engineers who designed the Nintendo Famicom in the early 80s. During initial design phase of the Famicom, one of the first things ...
20
votes
3answers
6k views
Where to buy a 6502 chip
I want to purchase a 6502 40 pin CPU in order to verify if my 6502 is faulty, I've seen I can buy from Hong Kong but would prefer closer (to the UK), RS and CPC don’t seen to have any.
Most answers ...
49
votes
13answers
8k views
What languages are better fit for generating efficient code for 8-bit CPU's than C?
I found Why do C to Z80 compilers produce poor code? very interesting as it pointed out that C (which was designed to be an abstraction of a CPU for porting Unix) was not a very easy language to ...
4
votes
4answers
493 views
Would having the console's CPU help in emulation hypothetically?
If I had a hypothetical computer just like nowaday's computers, but it had an NMOS 6502, would it provide me with a better emulation experience than if I were to emulate the 6502 inside software? Also,...
