RDC
®
RISC DSP Controller
R8810LV
RDC Semiconductor Co.
Rev:1.2
Subject to change without notice
25
Bus Interface Unit
The bus interface unit drives address, data, status and control information to define a bus cycle. The bus A19-A0 are non-
multiplex memory or I/O address. The AD7-AD0 are multiplexed address and data bus for memory or I/O accessing. The
2
S - 1
S are encoded to indicate the bus status, which is described in the Pin Description table in page 6. The Basic
Application System Block (page 8) and Read/Write Timing Diagram (page 12) describe the basic bus operation.
Memory and I/O interface
The memory space consists of 1M bytes and the I/O space consists of 64k bytes. Memory devices exchange information with
the CPU during memory read, memory write and instruction fetch bus cycles. I/O read and I/O write bus cycles use a separate
I/O address space. Only IN/OUT instruction can access I/O address space, and information must be transferred between the
peripheral device and the AX register. The first 256 bytes of I/O space can be accessed directly by the I/O instructions. The
entire 64k bytes I/O address space can be accessed indirectly, through the DX register. I/O instructions always force address
A19-A16 to low level.
Data Bus
The memory address space data bus is physically implemented as one bank of 1M bytes. Address lines A19-A0 select a
specific byte within the bank. Byte transfers to even or odd addresses transfer information in one bus cycle. Word transfers to
Memory
Space
FFFFFH
0
1M Bytes
I/O
Space
0FFFFH
0
64K Bytes
Memory and I/O Space
FFFFF
FFFFE
2
1
0
1M Bytes
D7:0
A19:0
Physical Data Bus Models
(X)
A19:0
First Bus Cycle
D7:0
(X+1)
A19:0
Second Bus Cycle
D7:0
8-Bit Data Bus Word Transfers