How much RAM is used for Multi-Monitor Support on Iris 5100 Graphics?

Apple states:

'Dual display and video mirroring: Simultaneously supports full native resolution on the built-in display and up to 2560 by 1600 pixels on up to two external displays, both in millions of colours' (http://apple.com/au/macbook-pro/specs-retina/), but they don't state if a performance/memory hit would be encountered while using external displays.

Technically, how many bytes make up a pixel on a 64-bit OS? Is it 8 bytes per pixel? (8 bits * 8 bytes = 64-bits per CPU cycle).

I'm wondering if 8GB RAM will be enough for external displays while efficiently and comfortably running a heavy amount of programs and web browsing, considering Iris 5100 integrated graphics can take up to a max of 1024MB of System RAM, surely that can't be enough for the amount of pixels required on Retina (2560-by-1600 = 4 096 000 pixels) in-built display + 2x maxed out external displays (8 192 000 pixels)...which officially equals 12 288 000 pixels supported at any one given time (not taking into account the hidden 4K res ability), and I heard a third external monitor can be supported via the HDMI port as well which would take up more RAM for video memory. :/

If anyone has hooked up their Haswell (Oct-2013) rMBPs to an external monitor setup, could you please print screen your RAM usage from Activity Monitor; before and after the external monitor(s) were hooked up, thanks.

I'm on the edge of staying with 8GB or going with 16GB, and this is the deciding factor! Cheers.

dusk007 said:

It is usually 8 bit aka 1 Byte per color. Three colors makes 24 bit and then there is 1 byte for the alpha channel, not sure if the framebuffer has that.
Either way the framebuffer is tiny compared to current RAM sizes.

If you had only 2GB RAM in your system that would be enough to run such a monitor.
2560-by-1600 = 4 096 000 pixels
* 24 bit = 98304000
/8 = 12288000
/1024 = 12000 kB
= 12 MB

That is miniscule compared to 8GB = 8192MB.
In the 1024MB VRAM you can store 85 framebuffers for that display of yours and even with Triplebuffering you only need 3.

Noboy will detect any noticeable increase in RAM usage from multiple monitors even if they only have 4GB RAM.

Click to expand...

Wow, that's quite a small amount, it has been a few years since I last did pixel-memory calculations. That * 24 bit part seems very familiar (and I missed it), and makes sense with the RGB colours of any screen. Alpha channel for transparency makes sense too.

Buffering would just multiply the 12MB by how many buffers you have. e.g. OSs usually contain 3 buffers, does that mean you would have 36MB (12MB * 3 buffers) of RAM usage dedicated to the display? That's pretty small, I honestly thought the display would be using one of the most amounts of memory (but that's probably for applications storing graphic intensive tasks and objects into RAM - for quick access, which is why games and graphical applications such as Photoshop, 3DsMax and AutoCAD require vast amounts of memory to run smoothly, albeit 8GB is sufficient for today's consumer laptop), but your statements make sense. Thank you!