ST10R272L - CENTRAL PROCESSING UNIT
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External operand writes.
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Testing Branch Conditions immediately after PSW writes.
4.4
CPU special function registers
Special Function Registers (SFRs) maintain the system state information, supply the ALU
with register-addressable constants, control the system and bus configuration, multiply and
divide ALU operations, code memory segmentation, data memory paging, and access the
general purpose registers and the system stack.
Since all SFRs can be controlled by any instruction which is capable of addressing the SFR
memory space, a lot of flexibility has been gained without the need to create a set of
system-specific instructions.
Note however, that there are user access restrictions for some of the CPU core SFRs, to
ensure proper processor operations. The Instruction Pointer (IP) and Code Segment Pointer
(CSP) cannot be accessed directly, they can only be changed indirectly via branch
instructions.
The PSW, SP, and MDC registers can be modified, not only explicitly by the programmer, but
also implicitly by the CPU during normal instruction processing. Note that, any explicit write
request (via software) to an SFR, supercedes a simultaneous modification by hardware of
the same register.
Note
Any write operation to a single byte of an SFR clears the non-addressed
complementary byte within the specified SFR.
Non-implemented (reserved) SFR bits cannot be modified and will always supply a
read value of ’0’.
4.4.1
The system configuration register SYSCON
This bit-addressable register provides general system configuration and control functions.
The reset value for SYSCON depends on the state of the PORT0 pins during reset.
SYSCON (FF12h / 89h)
SFR
Reset Value: 0XX0h)
Bit
Function
XPER-SHARE
XBUS Peripheral Share Mode Control
‘0’: External accesses to XBUS peripherals are disabled
‘1’: XBUS peripherals are accessible via the external bus during hold mode.
XPER-
SHARE
VISI
BLE
CS
CFG
PWD
CFG
OWD
DIS
BDR
STEN
SSP
EN
ROM
S1
WR
CFG
5
4
3
2
1
0
11
10
9
8
7
6
15
14
13
12
rw
rw
rw
rw
rw
rw
rw
rw
STKSZ
SGT
DIS
ROM
EN
rw
BYT
DIS
CLK
EN
rw
rw
rw
rw
rw
