Memory model and size
Segmented (Intel x86 (64KB), IBM 370-XA (2GB)) vs uniform linear address space
8-bit (256B), 16-bit (64KB), 32-bit (1GB, 2GB, or 4GB), 64-bit (18PB = 1.8e+19B)
IBM S/360 April 1964: $1/B ($5.55/B in 2000 dollars) vs IBM PC RAM in early 2000: $0.60/MB