Questions tagged [assembly]
Assembly languages in general, of any architecture. Use with the particular processor’s tag as appropriate.
183
questions
5
votes
1answer
188 views
Z80 CPU and nested/reentrant NMI
According to multiple documentation sources, non-maskable interrupts (NMIs) can be nested (or reentrant) in a Z80 microprocessor.
This is, upon a /NMI signal pulse, the CPU will interrupt the current ...
1
vote
0answers
85 views
Timer B Interrupt occurs only once
I was following this tutorial on Commodore 64 timer interrupts and tried to replicate it (except all I was planning on doing was changing the border color.) I noticed that my interrupt only occurs ...
1
vote
1answer
166 views
Game assembled using RGBDS can't access cartridge RAM
Any writes I do to cartridge RAM on my test game assembled with RGBDS are silently ignored. I wanted to move the stack pointer to cartridge RAM to have more room in high ram to use as a scratchpad, ...
1
vote
1answer
182 views
How does RETN return from NMI on the Z80 in the event of nested NMI?
I was trying to learn how the Sega Master System works. The NMI occurs when the pause button on the console is pressed. So I wrote an NMI handler that makes a beep when the NMI occurs, then waits for ...
1
vote
1answer
220 views
Can the window only be moved once per frame on the Game Boy?
I wanted to cover the top part of the screen to use as a status bar for a game. My idea was to use the Game Boy's window feature to do so. But I ran into a problem: The window's size is fixed to be ...
1
vote
0answers
92 views
Getting a loading seam to work with 2x2 metatiles
I'm working on a prototype for a scrolling loading seam similar to Pokemon where the screen scrolls incrementally by 8 pixels with each direction button press, and new tile graphics are drawn off the ...
4
votes
0answers
228 views
Game Boy interrupt handler crashes unless I jump to it indirectly
I have an interrupt handler I created, which is meant to reduce the overhead of interrupts as much as possible. Setting up the interrupt handler is done while interrupts are disabled, of course. It ...
5
votes
1answer
207 views
Can I use cartridge RAM for the stack pointer?
My understanding is that the extra cartridge RAM banks at &A000-&BFFF are battery-backed SRAM that is preserved after powering off the game. If I'm making a simple game that doesn't have a ...
24
votes
11answers
10k views
Why weren't 80s arcade games programmed in C?
I know many arcade games from the 80s were programmed in 68000 assembly. This carried on probably well into the 90s, even though Motorola C compilers existed in the 80s. Why then weren't C compilers ...
8
votes
1answer
1k views
Why does the Z80 JP (absolute) instruction always take 10 states to execute?
I've looked at a few different websites and they all agree that a JP nnnn instruction (where nnnn is a 16-bit address) always takes 10 states to execute.
According to what I've read, all of the ...
6
votes
2answers
432 views
How can I detect signed integer overflow on the Game Boy?
Unlike a normal Z80, the Game Boy has no CALL pe/po, JP pe/po or RET pe/po instructions. I've been trying to figure out how to detect integer overflow on the Game Boy. (It doesn't have those ...
16
votes
3answers
1k views
6502 ORA not updating the accumulator
Unless I'm reading the manual incorrectly, the following should produce $11 when D404 = $10 (I'm turning on the gate for voice1 which has been previously set to triangle wave %00010000/$10)
00153 ...
15
votes
2answers
1k views
How to use "AND," "OR," and "XOR" modes for VGA Drawing
I've been trying to implement a blitter in MS-DOS, but before I do that I wanted to test the various graphics modes of the VGA graphics card. (I'm emulating using DOSBox.) According to this paper on ...
5
votes
1answer
334 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 ...
14
votes
1answer
2k views
How can I set up the Microsoft C compiler to make it prefer immediate-mode push instructions?
I started a project to get a better understanding on how to compile a game for Windows 3.x. I tried to set up the build workflow so that it produce the byte-exact clone of a great open sourced Win16 ...
9
votes
0answers
258 views
Undocumented ModR/M byte combinations
The reason I am asking this question is because I was trying to compile a list of all 8086 opcodes, regardless of whether or not they are documented. The list is just for fun, the documented opcodes + ...
4
votes
2answers
460 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 ...
4
votes
2answers
209 views
Looking for NEC RA87 “Relocatable Assembler Package”
I admit, I am pushing my personal envelope (punching above my weight, beyond my pay-grade etc. etc.) on the trailing edge of technology. So please be kind!
I would like to write new firmware for a NEC ...
1
vote
2answers
289 views
8086 assembly relative addressing issue [closed]
This tiny program puts a few A's into the text mode buffer of an ms-dos pc. But only works if the number of repetitions (CX) is loaded with a direct value. If I try to read that from memory, i.e. ...
16
votes
3answers
2k views
Why can't I invoke the next interrupt service by incrementing the AX register after calling the same interrupt?
I have two snippets of 8086 assembly code, both of which are supposed to do the same thing: make the mouse appear on the screen.
Show_Mouse:
push ax
mov ax,0 ;Reset Mouse
int 33h
...
4
votes
1answer
239 views
ZX Spectrum coordinates to bitmap conversion subroutine acting strange
I originally posted this question on stackoverflow, but I wrote a little program that converts coordinates stored in the b and c registers of the z80 into their corresponding memory address for the ZX ...
5
votes
2answers
354 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 ...
1
vote
1answer
76 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 ...
7
votes
1answer
586 views
The proper way to shift a byte using a Z80
In a nutshell, why can I not shift a byte using this way?
LD A, 11110000b ; I want this to be 00001111b
LD C, 4
SRA C
OUT (0), A ; doesn't give correct number
What is the proper way of shifting a ...
6
votes
1answer
361 views
How does an 8086 CPU remember the memory address where it should write back the operation’s result? [closed]
I have an instruction: ADD [BX][SI] + 5FFDH, EABFH and I want to know how it operates exactly on 8086 microprocessors. I've realized that this instruction ADD [BX][SI] + 5FFDH, EABFH, works in this ...
4
votes
2answers
260 views
Cannot assemble BASIC with high memory workspace (Z80)
This is a homebrew Z80 computer, using Z80, 64K RAM, EEPROM as well as SIO/0.
I am trying to run BASIC, however I need to set the workspace at 0xC000 or higher.
The code I am using is similar to https:...
6
votes
3answers
761 views
Apple II Toggles between 40 and 80 columns in assembly language (Apple IIc)
To switch from 80-column to 40-column display, what is the difference between the following two actions:
lda #$11
jsr COUT ; $FDED
and
sta $C00C ; 80coloff switch
In the first case, I do not ...
4
votes
1answer
236 views
Apple IIc : displaying HGR page 1 in AUX memory
I need to display HGR page 1 ($2000 to $3FFF) located in AUX memory (not in MAIN memory).
According to the "Apple IIc Reference Manual - Volume 1", page 45, I must first turn 80Store on by ...
6
votes
1answer
607 views
Why does the JSR operation not jump?
I am building a computer based on the 65C02S, and when stepping through the code, I see strange things happening.
This is my code:
SEI
CLD
LDX #$ff
TXS
JSR someRoutine
...
This translates to this ...
7
votes
1answer
204 views
How did macro-instructions and indirect addressing interact in IBM's MAP/FAP assemblers?
The MAP and FAP assemblers for the IBM 709/7090/7094 had a nice macro facility. I was surprised by its power. (Actually, I guess it is possible that most macro processors at the time were as ...
5
votes
3answers
577 views
Was there an analog of local labels in the IBM assembly language?
In the assembly language used in Unix on DEC machines (PDP-11, VAX), one can use numerical labels and refer to them with suffixes "b" and "f" meaning "backwards" and &...
7
votes
1answer
338 views
What could be the reason an `LD B, B` instruction was used in this busy loop?
The Game Boy game "Tetris" leaves a transmission interval of about a millisecond between bytes sent over the serial link, as suggested by the official Game Boy Programming Manual, to give ...
21
votes
3answers
2k views
Z80 to x86 asm translator?
8086 is source code compatible with 8080. Zilog Z80 extended Intel 8080 with:
An enhanced instruction set including bit manipulation, block move, block I/O, and byte search instructions
New IX and IY ...
5
votes
1answer
179 views
Why have both the ORG and LOC pseudo-operations in the SHARE/Symbolic/Fortran Assembly Programs for the IBM 704/709/7090/7094?
This is a fairly basic question, and I almost feel ashamed to ask it; I'm guessing it can be answered in a single sentence.
There was a rather influential series of assemblers for some of IBM's ...
14
votes
1answer
1k views
Who are the people associated with Simula, Assembler and Fortran in this video?
I am watching a recording of a discussion panel ‘Unix50 - Unix Today and Tomorrow’, part of which contains Bjarne Stroustrup’s talk ‘From C to C++’, discussing the history of C++.
At 20:46 there is a ...
6
votes
3answers
2k views
Z80 string iteration failing
First Z80 project after 6502, a simple looking piece of code giving me hard time.
print_char is a working function which prints contents of A to the screen.
My first issue is that the following code (...
10
votes
0answers
339 views
Assembler where output file was executable WHILE including the source code
I seem to remember using an assembler with the following interesting workflow: You'd write an assembly language file and assemble it. The output overwrote your source code by placing the executable ...
2
votes
2answers
114 views
PDP-1 skip group invert bit functionality
I'm confused on the behavior of setting the invert bit on the PDP-1's SKP group. The manual states:
The intent of any skip instruction can be reversed by making Bit 5 (normally the Indirect Address ...
14
votes
2answers
1k views
Origin of "arithmetic" and "logical" for signed and unsigned shifts
The assembly language for many processors use the phrase "arithmetic shift" to represent the bitwise shift of a signed value, and "logical shift" for an unsigned value. The two ...
5
votes
3answers
258 views
Apple II: ProDOS, zero length prefix and MLI calls
I have a simple program on my Apple IIc written in assembly language. It starts by getting a filename from the user (just the file name, not the full path), then it opens the corresponding file with a ...
13
votes
3answers
2k views
What 8086 instructions accept REP?
I tried this code in my assembler, set to 16 bit mode:
bits 16
rep mov ds, ax
Surprisingly, no error was thrown. Is this even valid? Wasn't rep only supposed to work with string instructions? Is it ...
23
votes
12answers
7k views
Was memory corruption a common problem in large programs written in assembly language?
Memory corruption bugs have always been a common problem in large C programs and projects. It was a problem in 4.3BSD back then, and it's still a problem today. No matter how carefully the program is ...
0
votes
3answers
538 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
466 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
104 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
...
14
votes
1answer
2k views
How can a Z80 assembly program find out the address stored in the SP register?
I'm wondering how to write a program in Z80 assembler that discovers the value stored in the SP register.
From what I've seen, the only instructions that touch the SP register are:
LD SP, HL/IX/IY
EX ...
9
votes
3answers
1k views
Make the "z80asm" assembler place an instruction at a known memory address
I'm writing a very basic OS for my homebrew Z80 computer. As an absolute assembly language beginner, I managed to obtain a working "os plus memory monitor" that can show memory content and ...
0
votes
3answers
263 views
c64 copy chars to sprite issue
How to do correct copying char to sprite ?
Lets say 1st char is at $2000 second is at $2008 and copying to sprite at $2480.
And with this we can copy 1 char to top left position of 1st sprite. But ...
22
votes
1answer
1k views
When did the IBM 650 have a "Table lookup on Equal" instruction?
In 1959, Donald Knuth wrote an assembly program named SuperSoap for the IBM 650. Here is the manual, and here is a listing of the program (in SuperSoap assembly language). Quoting from the abstract:
...