"Hardware description of the 8051 microcontrollers" - читать интересную книгу автора (Hennig-Roleff W.)

To support computing, controllers and processors are equipped with 8 bit wide
data lines. Newer, faster CPU's in future will hold 16 or 32 BIT wide data
lines. With all this, the ASCII table is in addition used for the coding of
text (see Appendix). The following designations are normally used:

BIT 1 data line
NYBBLE 4 data lines = ы BYTE one digit hex value 0..0F
BYTE 8 data lines two digit hex value 0..0FF
WORD 16 data lines four digit hex value 0..0FFFF
DWORD 32 data lines eight digit hex value 0..0FFFFFFFF


MSB LSB
+ЧЧЧЧЧЧ+ЧЧЧЧЧЧ+ЧЧЧЧЧЧ+ЧЧЧЧЧЧ+ЧЧЧЧЧЧ+ЧЧЧЧЧЧ+ЧЧЧЧЧЧ+ЧЧЧЧЧЧ+
| Bit7 | Bit6 | Bit5 | Bit4 | Bit3 | Bit2 | Bit1 | Bit0 | BYTE
+ЧЧЧЧЧЧ+ЧЧЧЧЧЧ+ЧЧЧЧЧЧ+ЧЧЧЧЧЧ+ЧЧЧЧЧЧ+ЧЧЧЧЧЧ+ЧЧЧЧЧЧ+ЧЧЧЧЧЧ+
| High NYBBLE | Low NYBBLE |

MSB = most significant Bit
LSB = least significant Bit

2. Data and Addresses Page 3
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The 8051 is an 8 bit computer, that is: numeric values, characters and also
the code are stored as bytes. The data bus has 8 parallel data lines.

On chip, the data is transferred over an interconnecting INTERNAL DATA BUS.
If a processor board is constructed and an external RAM and/or EPROM is used,
then 8 connecting lines must be brought in between the 8051 and the external
memory for the data. This is the EXTERNAL DATA BUS.
The transition switching between the internal and the external data bus
takes place over special drivers (see ports).



In order to store a number the size of a BYTE, 8 flip flops will be needed.
To store a complete text, many such 8 flip flop memory locations will be
needed. In addition, each memory location should be individually accessible.
For each memory location a select line is needed.

Memory chips produced today are highly integrated (a 62256 RAM chip has
for example 32768 memory locations: it can store 32 kBytes). In order to
access a given memory location, it is not necessary to have 32768 select
lines. Instead, coded addresses are used, which are decoded only in the
RAM chip. So with 16 address lines, which the 8051 prepares, 65536 memory
locations can be addressed.


To select which memory location will be accessed on the 8051 depends