31. When the processor needs to access external memory, it starts placing the address of the requested information on the address bus . 32. For example, a processor with a 16-bit external address bus can only address 2 16 = 65536 memory locations. 33. Since the Z80 has a 16-bit address bus , it can only address 64 kB of memory at a time. 34. The processor address bus was now 16 bits and the instruction set consisted of 8, 16, and 24 bit instructions. 35. The 6100 has a 12-bit data / address bus , limiting RAM to only 4K words ( 6 KB ). 36. The external interface consists of a 128-bit data bus and a 43-bit address bus operating at 150 MHz. 37. It is actually two buses ( not on the diagram ) : an address bus and a memory cells using the data bus. 38. A further extended version which exposed 31 bits of the address bus was also produced, in small quantities, as the 68012. 39. Gate-A20 can be enabled or disabled by software to allow or prevent the address bus from receiving a signal from A20. 40. The biggest difference is that the 6545 may be configured so that it has sole access to the address bus for video memory.
