The PlayStation 4 reserves approximately 3.5GB of its 8GB GDDR5 RAM for the operating system according to a new report from Digital Foundry, which revealed that developers now have access to only 4.5GB of that 8GB GDDR5 for game code. According to the unnamed sources, a further 1GB of the 8GB GDDR5 is also suggested to be of "flexible memory" and may be reclaimed from the OS reservation, based on availability.

Sony's internal docs say that 4.5GB is the baseline amount of guaranteed memory available for game-makers (note the memory usage of the Killzone: Shadow Fall demo) and most likely what the lion's share of launch titles will be using. However, other sources close to Sony indicate that developers can request up to an additional gigabyte of "flexible memory", and use it to boost elements of the game - but only if the background OS can spare it. We're told that incorporating this isn't trivial, and it may well be that to begin with only first-party developers target its usage.

According to Digital Foundry, the current PlayStation 4 dev kits have what is being called a "Game Memory Budget Mode" in which the debut settings feature two options: normal and large. Those who choose to code in the normal mode will find the reported 4.5GB of GDDR5 RAM is usable for game code, however those choosing to code in the large mode apparently get an increase to 5.25GB of GDDR5 RAM. The high increase of RAM in large mode is apparently available only for application development, "presumably in order to house debugging data. From what we understand, the extra gig of flexible memory appears to work in addition to these allowances."

However, as Digital Foundry points out, the PlayStation 4 was originally suppose to support a 4GB GDDR5 system, with 512mb of that reserved for OS. The surprise announcement of a increase to 8GB of GDDR5 RAM obviously allowed Sony to offer more in the way of OS interaction, something that Microsoft has been showing off heavily with the Xbox One. A number of features including pausing games and switching to other applications instantly have been touted by both manufacturers, so its easy enough to see where the extra memory is going.

As it currently stands, those looking at both consoles will find that the Xbox One and PlayStation 4 will be launching with 8GB of unified memory, however both will presumably hold a certain amount of that RAM for the OS. It is unclear exactly what will happen in the future with these new consoles, but looking at Sony's previous trackwork with the PlayStation 3, we could see a decrease in the OS footprint in future revisions.

For more on this newly reported discovery, check out the Digital Foundry article over here on Eurogamer.