Questions tagged [6502]
For questions about the 6502 series of processors, including hardware and assembly language questions. For questions about the hardware interface also use the "hardware" tag.
79
questions
6
votes
2answers
189 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
703 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 ...
27
votes
2answers
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/...
11
votes
3answers
2k views
6502 reset pin: always needed on power on?
The 6502 pinout has a reset pin, which presumably would be used if a reset button is pressed on the machine containing the CPU.
But the 6507 contains the same pin. The 6507 was designed at the ...
3
votes
2answers
143 views
6502 interrupts when RDY is low
If the NMI line goes from high to low when the RDY is low, is the NMI detected, so that when RDY goes high, NMI is performed ?
Or is the NMI discarded when it occurs when RDY is low ?
(In case of ...
4
votes
2answers
611 views
Adding a register to the accumulator in 6502
In the past, the only assembly programming I've done has been a tiny bit of ARM, and then a larger game project in 68k.
In 68k I was used to being able to do things like add.l d0, d1 to add the ...
5
votes
3answers
756 views
Indirect addressing using address bits as marker (tagged address)
After reading about the Signetics 2650, and being a bit familiar with the HP2100's, I'm curious how common the style of "indirect addressing via the address value" instruction was at the time?
CPUs ...
15
votes
1answer
2k views
How did the 6502 ALU perform a decrement?
Assuming that this diagram is correct:
Instructions like INC, INX, and INY can easily perform increment using ALU sum with data on B input, 0 on A input and carry_in set.
But how do instructions ...
6
votes
1answer
174 views
Putting code into two different memory areas with cc65/ca65
According to cc65 / ca65 documentation, the .org directive does not affect the placement of the code unlike with other 6502 assemblers. The docs suggest defining a memory segment with the intended ...
20
votes
2answers
2k views
Why didn't the 6502 have increase/decrease opcodes for A?
In 6502 Assembly, we can use INX and INY to increase the value stored in X and Y. They can be decreased with DEX and DEY. However, it seems that there are no such instructions for A, like INA or DEA. ...
18
votes
4answers
3k views
Why are branches relative in many 8-bit CPUs?
I was looking over an old article on the 6809 and was perusing the opcodes and noticed that the branch instructions came in two flavors, long and short. That sparked a memory about one of the 6502-...
11
votes
1answer
394 views
Why didn't Wozniak's interrupts work on the Apple I monitor program?
In his autobiography, Steve Wozniak recounts[1] his difficulty getting interrupts working on the 6502 microprocessor:
The next step was to debug the 256-byte monitor program on the PROMs. I spent ...
39
votes
3answers
5k views
What happened to the SEV instruction on the 6502?
The 6502 has a group of opcodes which copy bit 5 from the opcode into one of the status flags.
(I know it's not implemented this way, but it looks as though the bit fields are: 2 bits to select the ...
15
votes
6answers
5k views
How slow was the 6502 BASIC compared to Assembly
Imagine a modern computer, where let's say Python is a high level programming language and needs to be interpreted in order to execute a piece of code. You could write some code in C, compile it, ...
14
votes
12answers
4k views
Late 1970s and 6502 chip facilities for operating systems
I'm quite unfamiliar with how operating systems used to be written but its clear to me that operating systems were clearly written for these systems. I'm particularly interested in how operating ...
24
votes
3answers
3k views
What are uses of the byte after BRK instruction on 6502?
The BRK instruction on the MOS 6502 seems to be one of the more
ill-documented features of the processor. The 1976 preliminary data
sheet from MOS indicates that it's a 1-byte instruction using
the "...
13
votes
1answer
737 views
Does the Commodore CDTV-CR contain a 65C02 for some reason?
Browsing around the FS-UAE source code I discovered a 65(S)C02 emulation core, which appears to not be referenced anywhere outside of some CDTV-CR related source file.
Apparently, a CDTV is ...
5
votes
4answers
614 views
6502: is BCD *fundamentally* the same performance as non-BCD?
Furthering my reading of MS BASIC and Woz's FP code and comparing the two leads to another question specific to the 6502...
From what I can see looking over the 6502 instruction guides, it appears ...
13
votes
3answers
499 views
Did any micros use the 6502 BCD mode in their OS?
I know the Atari's FP package used BCD for rather dubious reasons, but does anyone know of other examples of basic "operating system" level code on common platforms that used BCD?
I suspect BCD was ...
9
votes
4answers
616 views
How is PETSCII used by assembly-language programs?
First, a bit of context. I wrote an
interactive disassembler for 6502 code that,
among other things, attempts to identify ASCII strings automatically.
Appropriate source code for different assemblers ...
3
votes
3answers
274 views
Interrupt pin type on the 6502
I have a really basic question. I have to design an 65C02-based personal computer for a school project. I have to draw my own footprint for the 65C02. My question is what type of pin is the IRQB (...
9
votes
1answer
260 views
SYNC and the 65CE02 instruction timing
From the Wikipedia's 65CE02 page:
Internally, the pipeline of the 65CE02 was redesigned to reduce the
number of cycles required to execute an instruction. The 65CE02 can
recover faster from ...
8
votes
3answers
1k views
Behavior of the zero and negative/sign flags on classic instruction sets
It seems to me that there's effectively two ways that the zero bit could work.
Z is set iff the result of a computation is mathematically equal to 0.
Z is set iff a bit pattern consisting entirely of ...
7
votes
1answer
306 views
When did the 65C02 become available?
The Wikipedia article on the 65C02 is missing an introduction date for any of the mentioned models. I did a bit of poking about with no success. Does anyone know when they were introduced?
22
votes
1answer
951 views
How good was Woz's FP code?
I just came across this amazing 1976 article by Woz. In it he describes a relatively complete floating-point system for the 6502 with a 32-bit format (similar to earlier MS code).
I understand the ...
2
votes
2answers
349 views
How exactly do all control signals in 6502 work?
I'm trying to make a 6502 replica in Logisim. I want to know what exactly each control signal in 6502, how the clock cycles work and additionally I would like to see an example of these control ...
31
votes
5answers
5k views
Why does the 6502 have the BIT instruction?
The 6502 has a bit instruction which
copies two of the bits into the N and V flags,
pretends to and the byte with the accumulator, but discards the result and only affects Z.
I'm having a hard time ...
4
votes
1answer
172 views
65C816: Inputs TTL-compatible?
New 65C816 datasheets say that Vih (for the data bus in particular) is 0.8Vdd, i.e. not TTL-compaible.
Old data sheets (for example, the one published in Apple IIgs Hardware Reference) say that at 5V ...
9
votes
1answer
786 views
Looking for a specific 6502 Assembler
I found some 6502 assembler code for a Commodore 64 program that uses meta commands like #include "foobar", define the address to be assembled to via *=$1234 and define labels without a colon. ...
1
vote
6answers
1k views
Compile the 6502 assembler source code for running on modern computers [closed]
I want to make an executable file for modern computers that you can download and run without any other software, using any kind of a 6502 assembler.
Can I compile source code, written for the 6502, ...
7
votes
2answers
345 views
Could a 6502 use different speeds of RAM in a single machine?
I'm not sure I've entirely formed the idea in my head, but I'll ask anyway...
One of the things I learned here in RO is that the BBC gained speed by using faster RAM, thereby avoiding cycle stealing ...
9
votes
1answer
331 views
What's the purpose of HOLD command in 6502 stack pointer
In the Block Diagram the Stack Pointer Register (S) has a HOLD command in addition to the LOAD and BUS ENABLE commands. The HOLD command is linked to the S/S output of the Random Control Logic. What ...
3
votes
2answers
343 views
How to express hi/lo byte of a label in crasm
I am trying to move my project from xa (which I found rather buggy) to crasm, which is the other 6502 assembler that comes with debian.
My project contains a lot of lines like
ldx #<pname
ldy #&...
15
votes
5answers
3k views
Why are different emulators needed to run platforms that use 6502 assembly code?
To my knowledge, an emulator turns machine code for a console into something that your computer can understand. For example, assembly code from a Gamecube game is not the same as from a .exe file.
...
9
votes
1answer
1k views
Why does this 6502 code push a function address onto the stack before calling?
I am learning how to program the Atari 800 by examining some tutorial code that came with the IDE/Assembler I am using. I am using the MADS assembler for this.
putchar_ptr = $346
csrhinh = 752
...
16
votes
2answers
5k views
What does “jmp *” mean in 6502 assembly?
Right now I am learning 6502 assembly. Currently I am using the MADS assembler to program for the Atari 800. This program is just a small tutorial program that came with the assembler zip file I ...
4
votes
1answer
320 views
What is the format of a TurboGrafX instruction (modified 6502 instruction set)?
What is the general format of TurboGrafX processor instructions?
For example, and this is made up: one word for opcode + two words for flags + one word for every parameter listed under that ...
1
vote
2answers
563 views
Resource for 6502 assembly directives?
I am currently learning about 6502 assembly. One of the documents I am learning about it from is a super mario bros. disassembly document. When I was reading the code I noticed part of the code had ...
8
votes
3answers
3k views
6502 CMP instruction doesn't compare as expected
I am trying to code in 6502 assembly and for some reason the CMP instruction doesn't work. For example:
CLD
LDY #$03
LDA #$00
LDX #$05
CMP Y
BEQ Equal
STX $0200
Equal:
BRK
...
1
vote
2answers
340 views
Acorn System 1 - is it a collectable? [closed]
I have one of these things gathering dust at home:
Acorn System 1
I have the original manual (when I find it?) too.
It did work, the last time I powered it up, 35 years ago.
Should I throw it in ...
6
votes
2answers
532 views
Hardware multipliers on classic CPUs
Can anyone offer some back-of-envelope sort of estimates of what sort of gate count was needed to add a multiplier to 8-bit CPUs?
I ask because I find it a bit odd that the 6809 had one, but none of ...
16
votes
2answers
1k views
6502 branch offset calculation
This question expands on How does the 6502 implement its branch instructions?
I'm working on a cycle accurate VHDL implementation on an FPGA. I have much of the program logic already written, but I ...
7
votes
1answer
238 views
DEX: How does a 2-Cycle instruction affect RCL outputs AFTER it completes?
I (as so many people before me) am developing a software simulation of the 6502, mostly based on the often cited block diagrams and several other sources of information.
Basically all the components ...
5
votes
1answer
581 views
Fastest way to write 0x00 to a zero-page memory location? (6502, Atari 2600)
I have a kernel in my 6502 game that writes two dots to a sprite, so for example:
........
........
.XX..XX.
.XX..XX.
........
........
Either 0, 1, or 2 dots can be on at a time. This is done one ...
10
votes
3answers
1k views
Why do they use CLI in an IRQ routine?
I am studying an FLI routine in http://codebase64.org/doku.php?id=base:fli_displayer and I noticed that the CLI instruction is being used in this IRQ routine:
irq0: pha
...
cli ; &...
6
votes
3answers
3k views
How can a Z80/8080/6502 use VGA?
I've come across a couple of projects that manage to get VGA working with the Z80 and similar CPUs:
Veronica (6502 CPU, VGA signal generation by AVR)
ZC160 (Z80 CPU, VGA signal generation by GAL)
c-...
14
votes
7answers
1k views
New 6502 BASICs?
I sort of "have a thing for BASIC" right now, which has led to a couple of great exchanges here in RSE about the variations "back in the day". I'm wondering if anyone is aware of other modern BASICs ...
3
votes
3answers
1k views
Reading both keyboard and joystick with non-KERNAL code on C64
I'm implementing non-KERNAL joystick and keyboard controls for my Commodore 64 game.
I used to use the KERNAL SCNKEY routine earlier to read keyboard and it worked well with my joystick port 2 code. ...
7
votes
3answers
749 views
Was there any particular reason the 6502's LDX#imm and LDY#imm aren't opcodes A8 and AA?
Parts of the 6502 opcode map are fairly regular, and parts of it aren't. One bit that strikes me as really weird is the decoding of LD_ instructions, in relation to the decoding of other ALU ...
9
votes
2answers
380 views
Can 6502 programs be transliterated to the SPC700?
The SPC700's instruction set is derived from the 6502's, but moves the instructions around and adds a few. But is it fully backwards compatible?
Assuming that the program fully separates data and ...
