Questions tagged [assembly]

Assembly languages in general, of any architecture. Use with the particular processor’s tag as appropriate.

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Why would a NES game use an undocumented 1-byte or 2-byte NOP in production?

Reading the NESdev wiki page on CPU unofficial opcodes, I see a few games use an undocumented 2-byte NOP instuction in production: Puzznic, F-117A Stealth Fighter, and Infiltrator use $89 #i. Beauty ...
JAL's user avatar
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87 votes
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Why is the processor instruction called "move", not "copy"?

Many processors have an instruction called "move" (sometimes spelled MOV) which copies data from one location (the "source") to another (the "destination") in registers and/or memory. It does not do ...
JoelFan's user avatar
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72 votes
3 answers
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Why did DOS use dollar-terminated strings?

According to a few tutorials I am seeing, DOS used dollar-terminated strings to write to the terminal. This seems to also be documented here on INT 21H. AH = 09h - WRITE STRING TO STANDARD OUTPUT ...
Evan Carroll's user avatar
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65 votes
13 answers
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Were later MS-DOS versions still implemented in x86 assembly?

Recently, Microsoft published the source code of old MS-DOS versions on GitHub. What is odd in my opinion is the use of x86 assembly language for everything. The assembly language would not be my ...
juhist's user avatar
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64 votes
1 answer
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How did the SNES do the “pixelate” transition effect?

This effect is seen in many SNES games, including Super Mario World. The effect pixelates the screen, and makes the pixels larger, then smaller again when it switches to another scene. It is done so ...
Chewie The Chorkie's user avatar
63 votes
7 answers
12k views

Why did the MS-DOS API choose software interrupts for its interface?

Access to the DOS API was done through the INT 21h x86 instruction. This was always counter-intuitive to me, coming from 8-bit systems that accessed system services by calling subroutines through a ...
Brian H's user avatar
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61 votes
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Are there any modern compilers that can generate Amiga/m68k executables?

I would quite like to resurrect my openkick project. As I note on that project's README.md, it is stalled because GNU GCC is not fit for purpose. Sadly, there do not seem to be any other modern ...
pndc's user avatar
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54 votes
12 answers
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How was the first assembler for a new home computer platform written?

How did the 8-bit computers (Spectrum, Commodore, Atari, Amstrad etc) typically "bootstrap" from bare electronics into a platform with a working assembly language and OS? What I mean: An assembler is ...
SF.'s user avatar
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54 votes
7 answers
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Why use static RAM addresses instead of the stack?

I'm studying the 65c816 assembly for the 1994 game, Super Metroid. A hobbyist studied the game in-depth and created a RAM map. From it: 7E:0B56 - 7E:0B57 Moves Samus this distance horizontally, ...
Andrew Cheong's user avatar
48 votes
4 answers
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Why are first four x86 General Purpose Registers named in such unintuitive order?

On x86 the first four general-purpose registers are named AX, CX, DX, BX. It would be quite intuitive if their indices (those used in instruction encoding) were in alphabetical order, but instead of ...
Ruslan's user avatar
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What computer system is this from 1984 Doctor Who?

In the 1980s, the popular BBC science fiction series Doctor Who used some real computer system(s) to display impressive (for the time, OK, maybe not!) graphics. They actually ran the software real-...
CJ Dennis's user avatar
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41 votes
12 answers
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When did assembly source code begin to be written in lowercase?

In nearly all the assembly source files and published listings I read up to the early 1980s, the labels, mnemonics, and operands were written in all uppercase. Just a few years later, I noticed 80x86 ...
Paolo Amoroso's user avatar
40 votes
14 answers
9k views

What "unusual" syntax assembly languages are/were there?

This question may have the apparent form of a question soliciting a "list" answer, but I'm expecting the list to be very short, so please bear with me. What "unusual" syntax assembly languages are/...
dave's user avatar
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Who is credited for the creation of Assembly Language?

The C Programming Language was originally developed by Dennis Ritchie who also co-designed the Unix operating system with which development of the language was closely intertwined. Is there a ...
jwzumwalt's user avatar
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3 answers
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Why are assemblers called assemblers?

We tend use the term assembler as if there is a fundamental implication of being different from a compiler. This disparity is usually taken as a fact, no matter how sophisticated or primitive either ...
Raffzahn's user avatar
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37 votes
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When and why did high-level language compilers start targeting assembly language rather than machine code?

From what I've read, the first FORTRAN compiler built a machine-code program entirely in memory; it was, in fact, designed to read the entire source code of the program, and then sequentially load ...
supercat's user avatar
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33 votes
6 answers
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Which was the first programming language that had data types?

Machine language (and Assembly language) don't have the concept of data types, so if you want to add an int and a float variables in Assembly, you have to use the appropriate Assembly instruction that ...
user11869's user avatar
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31 votes
6 answers
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Managing registers/memory effectively on the Z80

I've been writing assembly for the Z80 for some time, lately. Initially, I had no experience with working with 8-bit processors, but after going through a tutorial for my platform and spending a lot ...
v-rob's user avatar
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31 votes
3 answers
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Why didn't the 6502 have increment/decrement 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. ...
LuNa's user avatar
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29 votes
9 answers
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How did early x86 BIOS programmers manage to program full blown TUIs given very few bytes of ROM/EPROM?

I've always wondered how so much functionality and relative luxuries(CMOS Configuration Utilities. See: https://geekprank.com/bios/ for an example) included with most popular x86 BIOSes could be ...
video_error's user avatar
26 votes
3 answers
4k views

Why are the Intel 8080's rotate instructions called opposite to intuition?

The Intel 8080 has two pairs of instructions for rotating the value of the accumulator -- RLC/RRC and RAL/RAR. One of them shifts the 8 bits in the accumulator circularly, writing out into the carry ...
Danya02's user avatar
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24 votes
11 answers
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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 ...
Synthetix's user avatar
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24 votes
5 answers
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When did Nintendo switch from assembly to a higher programming language?

I am only talking about games developed by Nintendo itself. I know that NES Super Mario Bros. was written in assembly. (Here is an SE answer saying the same.) They must have switched to a higher ...
zomega's user avatar
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24 votes
4 answers
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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-...
Maury Markowitz's user avatar
24 votes
5 answers
3k views

How much benefit should be expected on a more advanced compiler for z80/r800 based computers?

I am self studying about compilers, and get hands on very good textbooks about the subject. I am thinking in develop a compiler using the almighty llvm infrastructure to cross compile to old computers,...
flavio's user avatar
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24 votes
1 answer
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Where can I find the debate around snow-free screen updates mentioned in the Zen of Assembly Language?

Michael Abrash's book Zen of Assembly Language: Volume I (1989) contains a reference to a column in a computer magazine, though the actual source is not cited. From the text: I recall a debate in the ...
640KB's user avatar
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23 votes
12 answers
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 ...
比尔盖子's user avatar
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23 votes
2 answers
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What are these tiny TSRs doing?

I've been puzzled by this for a while now. The (very old) game Phantasie comes with three small TSRs that are run prior to running the main game executable. This is the content of the file PH.BAT, ...
db2's user avatar
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22 votes
5 answers
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Will PC-DOS run faster on 4 or 8 core modern machines?

When I run PC-DOS on my 4 core AMD Phenom chip, does it take advantage of the extra parallel CPU's? If not, is there a way to coax DOS to use all available CPU's or does this require specific ...
jwzumwalt's user avatar
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22 votes
3 answers
4k views

Was there any automated unit testing prior to 1972?

Prior to 1972, was there any automated unit-testing? Did any assemblers include a testing a framework? I can find some academic journals on this topic from the late 70s -- most of which I don't have ...
Evan Carroll's user avatar
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22 votes
3 answers
6k views

Why are these DOS console drivers wasting precious bytes?

While doing some research on DOS device drivers, I took a peek at the console drivers DISPLAY.SYS and ANSI.SYS that are part of the DOS 6.20 installation. Both have "Microsoft" stamped on, ...
Sep Roland's user avatar
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22 votes
3 answers
6k views

What makes MOVEQ quicker than a normal MOVE in 68000 assembly?

I'm "re-learning" 68000 assembly language and came across the "MOVEQ" command that is labeled "MOVE QUICK". According to the NXP Programmers Reference Manual (reference below), the command MOVEQ (...
cbmeeks's user avatar
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22 votes
2 answers
1k views

Where and when did the ".s" suffix for assembly-language source files originate?

The closest I was able to find on StackOverflow is What are .S files?, in which no answerer addresses why we use .s for assembly. (And .S for preprocessor/macro assembly; and gcc -S to produce ...
Quuxplusone's user avatar
22 votes
1 answer
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: ...
texdr.aft's user avatar
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21 votes
8 answers
11k views

Uses for the halt instruction?

What was the halt instruction in early CPUs such as the Z80 and 8080 used for? Here's a description of the Z80 instruction: The HALT instruction suspends CPU operation until a interrupt or reset ...
Jet Blue's user avatar
  • 1,995
21 votes
3 answers
3k 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 ...
Schezuk's user avatar
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21 votes
3 answers
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Why use repeated STZ instructions with the same operand on the 65C816 for the SNES (Super Nintendo)?

Please consider the code in Super NES Programming/Initialization Tutorial/Snes Init. Here is an excerpt: stz $2113 ; Plane 3 scroll x (first 8 bits) stz $2113 ; Plane 3 scroll x (last ...
AlphaCentauri's user avatar
20 votes
1 answer
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 ...
SZIEBERTH Ádám's user avatar
19 votes
12 answers
6k views

Did early assembly games use hardcoded memory locations?

In the era of C64, Apple][ GS, and SNES, did the games use hardcoded memory locations, or did they let the assembler help them (like modern assemblers)? If yes, how did they manage the memory?
Ignis Incendio's user avatar
19 votes
3 answers
2k views

How to use the "darker" CGA palette using x86 Assembly?

For those of you retro developers out there, I am having a hard time figuring out how exactly to select the "darker" color palette in 320x200 CGA mode. Here's what I have so far: setup_cga_graphics: ...
JohnnyStarr's user avatar
18 votes
6 answers
7k 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, ...
Bartek Malysz's user avatar
18 votes
3 answers
2k 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 ...
Simon's user avatar
  • 283
18 votes
4 answers
7k views

MSX Assembly/Basic programming documentation

I'd like to try to create software (and maybe simple games) for the MSX standard. I know I have two ways to do so, either I use MSX Basic or Z80 assembly. I think that Z80 assembly is more powerful ...
Informancien's user avatar
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18 votes
5 answers
5k views

Was Family BASIC for the NES/Famicom powerful enough to create full games and applications?

"Family BASIC or Famicom BASIC is the consumer product for programming Nintendo's Family Computer video game console of Japan." wikipedia I can't seem to find much info about Family BASIC ...
LateralTerminal's user avatar
18 votes
2 answers
3k views

Meaning of $ and $$ in Modcomp assembly language

The Modcomp II was a 16-bit minicomputer circa 1970. As well as in FORTRAN IV, the Modcomp family could be programmed in assembly language, with syntax like this excerpt from Kermit for Modcomp: ...
Wayne Conrad's user avatar
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17 votes
3 answers
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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 ...
puppydrum64's user avatar
  • 1,638
17 votes
2 answers
6k 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 ...
user115898's user avatar
17 votes
5 answers
2k views

Has there ever been a instruction set architecture that did not require instruction decoding at all?

I am studying basic principles of instruction set architectures and am considering what it would take to not have any instruction decoding at all. I.e., all the control lines of the computer would be ...
Gunther Schadow's user avatar
17 votes
3 answers
4k views

Does this 1978 code for a 6800 really clear ALL of memory?

On page 43 of the October 1978 Dr. Dobb's Journal (reprinted in Volume 3 Page 425 Issue 10 Number 29) is a letter to the editor giving three 6800 routines to clear ALL of memory, stating... Routine ...
Les's user avatar
  • 171
16 votes
12 answers
8k views

How can a Z80 assembly program find out its own memory address?

I'm wondering how to write a program in Z80 assembler that discovers its own memory location. I thought that maybe I could somehow load the program counter PC into, for example, BC. Is there a way ...
twisted's user avatar
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