As proof of this you can make a very quick check.
import pynq
import numpy as np
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# Declare test array, an 8-element array from 0 to 7
test_array = np.arange(0, 8, dtype=np.uint8)
#Assign test array to an 8-element (8-bit) pynq buffer
in_buff1 = pynq.allocate((8,), dtype=np.uint8)
in_buff1[:] = test_array
# Assign test array to a 1-element (64-bit) pynq buffer
in_buff2 = pynq.allocate((1,), dtype=np.uint64)
aux = 0;
for i in range(len(test_array)):
aux += test_array[i] * 2**(i*8)
in_buff2[:] = aux
# Compare byte representation (this is how the elements are stored in memory)
print("Byte representation equal: {}".format(in_buff2.tobytes() == in_buff1.tobytes()))
Hope this helps
Mario