Questions tagged [6502]
For 6502 series of processors, including hardware and assembly language questions. Use with [hardware] for the hardware interface in particular.
127
questions
6
votes
1answer
261 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 ...
5
votes
1answer
463 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
541 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 ...
13
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
164 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
336 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
815 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
1k 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 ($69) which adds the accumulator to an immediate value and ...
4
votes
2answers
438 views
How much memory does this SYM-1 have?
I have a SYM-1 with the following memory configuration:
How much memory is that? Those all appear to be L2114UCE chips, which are described as "Static 1024x4 NMOS RAM", but I'm not sure how ...
4
votes
1answer
534 views
Sym-1: What is the “f” register?
Running SUPERMON on a Synertek SYM-1, the R (REG) command cycles through a number of registers. Most are self explanatory:
p is the program counter (pc)
s is the stack pointer
a, x, and y are obvious
...
6
votes
1answer
159 views
Chaining IRQ's with BASIC & Kernal routines turned off
Having disabled BASIC and Kernal routines I set up an IRQ to run the following code when the raster line reaches 200:
.irq1
inc 53280 // change border colour
// lda #$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
167 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
355 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
418 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
88 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
...
5
votes
1answer
329 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
515 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 ...
9
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
279 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
276 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
492 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
277 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
212 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
453 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
286 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
280 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
514 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,...
57
votes
8answers
13k 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 ...
18
votes
3answers
5k 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 ...
46
votes
13answers
7k 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
424 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,...
8
votes
3answers
535 views
Did the 6502 need both ADC immediate and SBC immediate?
The 6502 uses the carry bit system (over a borrow bit). On such machines, the bitwise NOT is applied to the subtrahend before it is "subtracted" (really, added, taking advantage of two's complement) ...
11
votes
3answers
454 views
Do any 6502 assemblers have better support for self-modifying code?
I find I keep on doing things like this:
raster_interrupt:
bgnd_color = * + 1
lda #$00
sta $d021
; rest of interrupt handler
so that I can do this kind of thing elsewhere in my ...
9
votes
1answer
296 views
6502 False Reads and Apple II I/O
The 6502 series of CPUs has an
interesting quirk:
indexed loads and stores may perform a "false read", reading from the
target address or a different address on a previous page. There are
situations, ...
16
votes
3answers
1k views
For how long can you safely stop the clock on an NMOS 6502?
As with most NMOS processors, the NMOS versions of the 6502 (and even
earlier CMOS versions) do not have a static core. Thus, if you run the
clock too slowly or stop the clock for too long while doing ...
2
votes
3answers
201 views
How can I get cl65 to include its subroutines, like pushax and tosmulax?
This question is about cc65, which is a toolkit including a C compiler, assembler, linker, etc. All targeting various 6502 computers, like the various 8 bit commodores, the Apple II and whatever else. ...
3
votes
3answers
488 views
How to transfer the address of a data block into zero page using the Merlin 8 assembler on my Apple //e?
I've been fooling around with the Merlin 8 (v2.58) assembler on my Apple IIe and ran into a small snag.
Basically, I would like to load the address of a block of data into a two-byte pair in zero ...
15
votes
1answer
2k views
Why did the Apple IIe use a 2 MHz-rated 6502, even though it ran at only 1 MHz?
The Apple IIe was designed to be timing-compatible with the Apple II+ so that timing-depndent software (such games and copy-protection systems) and hardware designed for the Apple II and II+ would, ...
19
votes
4answers
3k views
How did integer multiplication work in 8-bit BASIC without CPU support
I've recently been teaching my 11 year old binary multiplication, which is on the UK maths syllabus at secondary school. We have used long multiplication, eg shift and add.
This has made me wonder ...
13
votes
0answers
548 views
Why did some MOS 6502 packages have holes in them?
Someone was recently selling an early MOS 6502 CPU on Ebay (date code 0277) in a ceramic package that had a couple of holes in it. You can see them clearly (under the tape, which presumably was not ...
4
votes
4answers
1k views
Why use an ADC or SBC with immediate zero on a 6502?
I'm perusing the Apple I version of BASIC, later known as Integer BASIC on the II. In several places I see...
ADC #$00
or
SBC #$00
Unless I'm mistaken, these mean add/subtract zero. What ...
7
votes
2answers
551 views
How to pass data pointer to subroutine to use with Absolute,Y addressing?
I may be going about this the wrong way, so sorry if it is wrong, I'm coming to 6502 after 32 years (when I was 8 years old !)
I want to pass a pointer to some data, either data1 or data2 to a sub ...
4
votes
1answer
822 views
6502 - AND instruction updates flags differently than other logic operations
Why AND instruction updates flags in the fetch step?
Others logic instructions like ORA and EOR update flags in the same step that they update accumulator, in the decode step.
Is it a bug of ...
31
votes
3answers
3k views
How did Elite do vertex transformation?
In 3D graphics, vertex transformation is the process of converting x,y,z coordinates in 3D space, to x,y coordinates on the screen. According to https://www.khronos.org/opengl/wiki/...
