What does the MACs(8x8) of Ethos-U55 specification mean?

Let me explain in more detail with considering 256 MAC config. 

256 MAC config means that can do 256 8x8 multiplications per cc. A MAC counts as two operations (mul+add).

Suppose you have a network of 8 bit IFM. for e.g

INPUT :  10 * 10 * 3

Convolution : 128*3*3*3 (3x3x3 (HxWxD) kernel and 128 filter) (Same pad)

Output shape: 8*8*128

MAC = 8*8*128*3*3*3 = ~221184 MACs

Now, 256 8*8 MAC can happen in one clock cycle. So, you would need 221184/256 = ~864 cc for computing these MAC's.

Now say if your IFM is 16 bit then this means that we can do 128 16x8 multiplications per cc. which mean that now we need 1728 cc for these MAC operations. 

You can compute things for other configs similarly. 

Note that there are other parameter like DPUs per configs and to get this maximum performance, we need to fill all DPUs inside MAC.Generally the width, height and depth should be a multiple of the microblock size which is 2x2x8 for Z-256. You can refer Arm U55 TRM for details.

Let me explain in more detail with considering 256 MAC config. 

256 MAC config means that can do 256 8x8 multiplications per cc. A MAC counts as two operations (mul+add).

Suppose you have a network of 8 bit IFM. for e.g

INPUT :  10 * 10 * 3

Convolution : 128*3*3*3 (3x3x3 (HxWxD) kernel and 128 filter) (Same pad)

Output shape: 8*8*128

MAC = 8*8*128*3*3*3 = ~221184 MACs

Now, 256 8*8 MAC can happen in one clock cycle. So, you would need 221184/256 = ~864 cc for computing these MAC's.

Now say if your IFM is 16 bit then this means that we can do 128 16x8 multiplications per cc. which mean that now we need 1728 cc for these MAC operations. 

You can compute things for other configs similarly. 

Note that there are other parameter like DPUs per configs and to get this maximum performance, we need to fill all DPUs inside MAC.Generally the width, height and depth should be a multiple of the microblock size which is 2x2x8 for Z-256. You can refer Arm U55 TRM for details.