Virtual machine code review | Rust Programming Language Community | Page 1

gloomy heron Sep 2, 2025, 5:50 AM

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i am writing a vm in rust so i just need code review am i going right or not .????

https://play.rust-lang.org/?version=stable&mode=debug&edition=2024&gist=e6bb0ee62cf70558d827e09e772bef42

Rust Playground

A browser interface to the Rust compiler to experiment with the language

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i am following the little endian format

vernal otter Sep 2, 2025, 7:20 AM

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#[repr(u8)]
pub enum OPCode{
    Nop=0x0001,
    LoadImm=0x0002,
}

that is a bit silly

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1 byte is 2 hexadecimal digits

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not 4

gloomy heron Sep 2, 2025, 7:22 AM

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i see

slender sierra Sep 2, 2025, 7:23 AM

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_=>Err(format!("uknown op code 0x{0:X}",ins))
``` this arm is unreachable (without going through undefined behavior)

gloomy heron Sep 2, 2025, 7:23 AM

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fixed

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i need a little help regarding the immedeate value

vernal otter Sep 2, 2025, 7:24 AM

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slender sierra ```rs _=>Err(format!("uknown op code 0x{0:X}",ins)) ``` this arm is unreachable ...

yeah the whole transmute is just direct ub

gloomy heron Sep 2, 2025, 7:25 AM

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like my instruction is load Ro , 15 so 8 bits are taken by op code and rest 4,4 by imm value and reg but i need to store more than 4 bit imm values

vernal otter Sep 2, 2025, 7:25 AM

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you should use the little endian utilities from std

gloomy heron Sep 2, 2025, 7:26 AM

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vernal otter you should use the little endian utilities from std

such as .???

vernal otter Sep 2, 2025, 7:26 AM

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instead of doing your own stuff with bit masks

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https://doc.rust-lang.org/std/primitive.u16.html#method.to_le_bytes

u16 - Rust

The 16-bit unsigned integer type.

gloomy heron Sep 2, 2025, 7:26 AM

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i see

vernal otter Sep 2, 2025, 7:26 AM

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https://doc.rust-lang.org/std/primitive.u16.html#method.from_le_bytes

u16 - Rust

The 16-bit unsigned integer type.

gloomy heron Sep 2, 2025, 7:26 AM

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any healp for this

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like my instruction is load Ro , 15 so 8 bits are taken by op code and rest 4,4 by imm value and reg but i need to store more than 4 bit imm values

vernal otter Sep 2, 2025, 7:43 AM

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also you really should not use as except maybe for enum casts, and even then probably not

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you code is a great example of why as is bad

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you freely mix truncating and non truncating casting with no sign of which is which

vernal otter Sep 2, 2025, 7:44 AM

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gloomy heron like my instruction is load Ro , 15 so 8 bits are taken by op code and rest 4,4...

ig reg could be 3 bits ?

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if you never add any more values

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otherwise you'll just need to use more bytes

gloomy heron Sep 2, 2025, 8:40 AM

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vernal otter also you really should not use `as` except maybe for enum casts, and even then p...

umm for what part are you specifically talking about

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also what should i do insted

vernal otter Sep 2, 2025, 8:48 AM

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gloomy heron umm for what part are you specifically talking about

you should remove all ass. maybe keep 1 in a functon specifically for that

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 let upper_bits = (byte & 0xff00) as u8;

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you got this

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that's litterally always 0

gloomy heron Sep 2, 2025, 8:49 AM

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yeah i fixed that

vernal otter Sep 2, 2025, 8:49 AM

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you should use from where it is available

gloomy heron Sep 2, 2025, 8:49 AM

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   let upper_bits = ((byte >>8) &0xff) as u8;

vernal otter Sep 2, 2025, 8:49 AM

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and other methods where possible

vernal otter Sep 2, 2025, 8:50 AM

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gloomy heron ```rust let upper_bits = ((byte >>8) &0xff) as u8; ```

that's just silly though

let [upper_bits, lower_bits] = byte.to_be_bytes();

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also calling byte a u16 is weird, but i shouldn't judge naming

gloomy heron Sep 2, 2025, 8:51 AM

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actually i am very confused about writing vm like how to impl these ive already implemeted chip -8 it was easy cause it had its docs and specification specified

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so i was able to think clearly

wide crown Sep 2, 2025, 5:53 PM

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is this a VM for a specific existing system, or are you designing your own ISA and system?

#Virtual machine code review