Epson ARM.POWERED ARM720T - Manual (Page 128)

8: Coprocessor Interface
8-8 EPSON ARM720T CORE CPU MANUAL
8.4.7 Coprocessor load and store operations
The coprocessor load and store instructions, LDC and STC, are used to transfer data between
a coprocessor and memory. They can be used to transfer either a single word of data or a
number of the coprocessor registers. There is no limit to the number of words of data that can
be transferred by a single LDC or STC instruction, but by convention a coprocessor must not
transfer more than 16 words of data in a single instruction. An example sequence is shown in
Figure 8-4.
Note:
•
The external coprocessor must not abort on LDC and STC instructions unless
they can be decoded as a CP15 operations otherwise dead lock occurs on busy
waiting.
•
If you transfer more than 16 words of data in a single instruction, the
worst-case interrupt latency of the ARM720T processor increases.
Figure 8-4 Coprocessor load sequence
ADD SWINE TST LDC SUB
TST LDC SUB ADD SWINE
LDC SUB ADD SWINE TST
I Fetch I Fetch I Fetch I Fetch CP data I Fetch I Fetch
(ADD) (SUB) (SWINE) (TST) (CPDO)
HCLK
Fetch
stage
Decode
stage
Execute
stage
CPnCPI
(from core)
(from coprocessor)
HRDATA[31:0]
n=4
EXTCPA
(from coprocessor)
EXTCPB
CP data CP data CP data