35
votes
Accepted
What stops an assembly program from crashing the operating system?
In the end, all programs are machine code, regardless of whether the source language was assembler or a high-level language.
The important thing is that there are hardware mechanisms that limit what ...
- 185k
34
votes
Accepted
Upload ASM code on Intel 8086 chip
The 8086 in itself doesn't have any memory, so you can't upload anything to it. You need to build a whole x86 computer with that chip in its core and then put a program in memory at the position the ...
- 110k
31
votes
Accepted
Fiducial marks: Do they need to be a pad or is it okay if I use the top silk layer?
You need to use a copper pad, with a corresponding clearance in the soldermask and ground planes.
The reason for not using the silk layer, is the tolerances of aligning the silk are typically ...
- 73.8k
22
votes
Accepted
How to program the CPU when making a small microcomputer?
You use an in-circuit serial programmer (ICSP), debugger, or JTAG that sits between your computer's USB port and your board. You lay out the board so the programmer connects to the board and ...
- 58.1k
20
votes
Accepted
What does [WEAK] mean in STM32 startup assembly code?
It says the implementation of the function should be weakly linked (as opposed to strongly linked, which is the usual).
This allows providing a "fallback" implementation of a function, in case no ...
- 16.4k
19
votes
How to program the CPU when making a small microcomputer?
Designing a board from scratch like this, with a CPU you haven't used before, can be difficult, and probably won't work.
When working with a new CPU, the safest approach is to get hold of an off-the-...
- 1,590
17
votes
How does an operating system or program detect the CPU model name?
For x86, you can use the CPUID instruction. It allows you to query various pieces of information about the CPU. It's built right into the CPU (and is not an operating system service).
- 20.8k
14
votes
Upload ASM code on Intel 8086 chip
You need to attach something to the CPU memory bus at specific starting location from where the CPU can fetch program code to execute.
For the 8086, it starts executing code from memory address ...
- 196k
12
votes
Accepted
Assembly code, what does it do?
These look like instructions for a RISC-V microprocessor.
https://riscv.org/wp-content/uploads/2017/05/riscv-spec-v2.2.pdf
ori is the "OR Immediate" instruction.
slli is a "Left Shift ...
- 13.5k
12
votes
Compile time error with inline PIC assembly in MPLAB XC16 C
This:
asm volatile ("BSET PORTB, #8");
should work if you are using the assembler from the xc16 compiler.
It's a syntax problem. The bit number is an ...
- 2,619
11
votes
Accepted
Why this code wasn't written in a much simpler way?
You are right in that it appears the code you show is silly. Perhaps whatever machine this runs on can't do immediate operations to set bits on I/O ports, and that that's why something like SETB P2.2 ...
- 317k
11
votes
What stops an assembly program from crashing the operating system?
Many multitasking operating systems use a data structure called a Process Control Block (PCB) to take care of the register overwrite problem. When you run your code, the OS creates a new process to ...
- 141
11
votes
Accepted
How does an operating system or program detect the CPU model name?
The architecture defines what results software can expect when running on an implementation conforming to the architecture. It is essentially a contract that ensures binary compatibility between ...
- 8,069
11
votes
Accepted
Logic value of a single input pin into a microcontroller
Generally, if the MCU has no mechanism of reading just one bit, you read the whole port and mask off the irrelevant bits you want to ignore to zero and then compare if the remaining bit makes the ...
- 196k
10
votes
Accepted
Why does __libc_init_array cause an exception?
Solution is to add -mcpu=cortex-m4 and -mthumb to your LFLAGS variable.
I'm not super-well-versed on GCC internals; please, someone correct me if I'm mistaken.
The reason the flags need to be added ...
- 2,556
10
votes
Accepted
How to produce a middle C on intel 8080?
DCR C decrements the C register and sets flags; it does not affect the accumulator (aka A register). The only instruction in this sequence which affects the ...
- 1,047
10
votes
Accepted
How can I do immediate addition in the Atmel AVR instruction set?
There is a subi instruction - "subtract immediate". You can easily do addition using this instruction, so one can speculate there isnt a need for a ...
- 73.8k
9
votes
Why this code wasn't written in a much simpler way?
The code is almost certainly for a processor using the 8051 instruction set. On that processor, the code variation you give would have the same effect as the original except that it would run faster. ...
- 47.6k
9
votes
Accepted
How to build the simplest assembly file in MPLabX IDE 5.4
Microchip has made it very to hard to develop 8-bit assembly language applications using the latest release of MPLABX v5.40.
To help I have crafted a PIC16F84A example project you can find here.
This ...
- 1,449
9
votes
Fiducial marks: Do they need to be a pad or is it okay if I use the top silk layer?
A fiducial must be in the top and bottom copper layers so it has perfect registration with respect to the rest of the component pads.
In our system a fiducial was a component (complete with a ...
- 26.2k
9
votes
Accepted
ARM How to invoke branching?
From ARM conditionals you can readily find that the instruction examines the Z, N, and V status flags and branches when Z=0 & N=V. Since it examines the V status flag and not the C status flag, ...
- 79.7k
8
votes
As a computer science engineering graduate, do I need to learn Assembly language in today's day and age?
Not knowing assembly is just fine UNTIL you have to debug something non trivial.
Interrupt trampolines, interrupt entry and exit stubs, missing memory barriers before cooking off a DMA transfer, ...
- 18k
8
votes
Accepted
What does _SFR_IO8(0x04) do? AVR
It is a macro to access AVR IO address space registers, and in that address space, register 4 seems to be DDRB. You might want to actually use the datasheet as the register reference, instead of ...
- 196k
8
votes
Logic value of a single input pin into a microcontroller
How can I get the value of the input pin P0.1?
You may "jump" on a bit value (e.g. JB, JNB): ISA
But this limits my action since I'm obligated to have all other pins of port P0 to L.
Not ...
- 11k
7
votes
Why this code wasn't written in a much simpler way?
The assembly code given is likely compiler-generated. It is the unoptimized version of the following C statements, where P2_2 and ...
- 260
7
votes
Accepted
Expression calculated at assembly time
The assembler (compiler) has to calculate the constant at assembly time, so the compilation to binary is marginally slower. A human cannot tell the difference so the compilation time it takes is ...
- 196k
7
votes
Upload ASM code on Intel 8086 chip
The 8086 loads an instruction from the Data memory space after it completes its power on and reset is deasserted. This instruction is the first instruction of whatever program or bootloader you want ...
- 93.2k
7
votes
Accepted
Are `call` and `return` usually instructions in a modern ISA?
The short answer for any reasonable definition of modern is "yes". Even little things like PIC16 have call and return. ARM, for historical reasons, spells CALL as "bl" and RETURN ...
- 47.4k
6
votes
Accepted
PIC programming in Assembly
de is an assembler directive to declare an EEPROM data byte. org is also a directive.
You can find a full list of assembler directives in the MPASM manual (see 4.17 for de).
In general all ...
6
votes
Help in understanding Store Word (SW) instruction in Risc-V
With RISC-V assembler, the operand order is destination/source except for stores.
Thus, your example instruction reads:
...
- 578
Only top scored, non community-wiki answers of a minimum length are eligible