Insufficient Memory for MAKEMAT(0,320,240)

[matalog]

I was trying to make a little graphics program, and have the calculator keep track of the pixels on screen, so I tried M1:=MAKEMAT(0,320,240) thinking that it wouldn't be a problem, but the calculator (G2) reported insufficient memory.

I imagined this would be 320x240 = 76,800 bytes for 8 bit numbers, maybe 76,800 x 5 for larger numbers. What is the initial expectation for the type of numbers in a Matrix?

Is there a way to be able to keep track of the numbers associated with each 320x240 pixels, I only need room for a 9 bit number really, so 16 bits would be fine.

The Hp Prime G2 has 256 MiB RAM or 268,435,456 bytes so I didn't expect this to be a problem.

Is M1:=MAKEMAT(0,320,240) the wrong way to use this?