Debugers have been already mentioned, but antiviruses do that, as well as various cheat and anti-cheat engines.
From a security perspective, reading/writing another program's memory space is well understood as a specific, risky privilege, so in all modern OSes control and prevent that if it's not "your" process or unless you're root/admin already.
But a cool thing for reading/writing another program's memory space is the ability to do that through PCI without the OS being able to intervene - e.g. Ulf Frisk's PCILeech (https://github.com/ufrisk/pcileech/), here's a video demo of writing to Notepad's memory remotely https://youtu.be/5DbQr3Zo-XY?t=1440
If I remember correctly, not long ago someone posted here about a cheat using DMA to modify the display buffer of a shooter. I'm in a hurry but I'll look it up later.
Well, with DMA, one can do pretty much anything. Doesn’t Thunderbolt (which is PCIe and hence has DMA) have this problem? As in, couldn’t a rouge TB peripheral siphon your data?
Thunderbolt & Firewire before it. DMA to external devices is powerful but the feature also means you expose a bus that wasn't originally conceived to be read by malicious agents.
From a security perspective, reading/writing another program's memory space is well understood as a specific, risky privilege, so in all modern OSes control and prevent that if it's not "your" process or unless you're root/admin already.
But a cool thing for reading/writing another program's memory space is the ability to do that through PCI without the OS being able to intervene - e.g. Ulf Frisk's PCILeech (https://github.com/ufrisk/pcileech/), here's a video demo of writing to Notepad's memory remotely https://youtu.be/5DbQr3Zo-XY?t=1440