Memory Map Sketch #1

Juno has a 16 bit address bus providing an address space of 64KB.

This address space is divided into 4KB blocks (defined by the 4 most significant bits) that the address decoder uses to select and therefore ensure only one device is driving the data bus at a time.

There are multiple devices connected to the data bus including ROM, RAM and IO devices. Typical ROM and RAM chips are much bigger than the 4KB block used and therefore multiple blocks will select the same  device. The reason for using 4KB blocks is that using a simple line decoder to implement the address decoder requires all memory blocks to be the same size and 4KB is more than sufficient space for mapping the IO devices into memory.

Memory Block	Dec	Hex	Memory Map	A[15-12]	A[11-0]
0	0	0	ROM	0000	xxxxxxxxxxxx
1	4096	1000		0001	xxxxxxxxxxxx
2	8192	2000		0010	xxxxxxxxxxxx
3	12288	3000		0011	xxxxxxxxxxxx
4	16384	4000		0100	xxxxxxxxxxxx
5	20480	5000		0101	xxxxxxxxxxxx
6	24576	6000		0110	xxxxxxxxxxxx
7	28672	7000	I/O	0111	xxxxxxxxxxxx
8	32768	8000	RAM	1000	xxxxxxxxxxxx
9	36864	9000		1001	xxxxxxxxxxxx
10	40960	A000		1010	xxxxxxxxxxxx
11	45056	B000		1011	xxxxxxxxxxxx
12	49152	C000		1100	xxxxxxxxxxxx
13	53248	D000		1101	xxxxxxxxxxxx
14	57344	E000		1110	xxxxxxxxxxxx
15	61440	F000		1111	xxxxxxxxxxxx
	65536	10000

NB a memory decoder for the above memory map can be implemented using a 4-to-16 line decoder to select devices e.g. 74HC138

More sophisticated memory decoders can be implemented using a memory device whose content maps addresses to devices allowing for much greater granularity. 

Even more sophisticated memory mapping techniques include bank swapping in which an external flag can be set to swap devices that are mapped to a specific part of the memory address space. This technique could be implemented in Juno using a bank swapping device that used IO commands to allow the selection between banks.

Much more sophisticated memory management techniques such as segementation, virtual memory and paging have not been considered.