ST9291 ST Microelectronics, ST9291 Datasheet - Page 13

• Manufacturer Part Number: ST9291
• Description: 16-48K ROM HCMOS MCU WITH ON SCREEN DISPLAY AND VOLTAGE TUNINGOUTPUT
• Manufacturer: ST Microelectronics

SYSTEM REGISTERS (Continued)
1.1.3.3 Register Pointing Techniques
Two registers, R232 and R233, within the system
register group, are available for register pointing.
R232 and R233 may be used together as a single
pointer for a 16 register working space or sepa-
rately for two 8 register spaces, in which case
R232 becomes Register Pointer 0 (RP0) and R233
becomes Register Pointer 1 (RP1).
The instructions srp, srp0 and srp1 (the Set
Register Pointer instructions) automatically inform
the ST9 whether the Register File is to operate with
a single 16-register group or two 8-register groups.
The srp0 and srp1 instructions automatically set
the twin 8-register group mode while the srp in-
struction sets the single 16-register group mode.
There is no limitation on the order or positions of
these chosen register groups other than they must
be on 8 or 16 register boundaries.
The addressing of working registers involves use of
the Register Pointer value plus an offset value given
by the number of the addressed working register.
When addressing a register, the most significant
nibble (bits 4-7) gives the group address and the
least significant nibble (bits 0-3) gives the register
within that group.
REGISTER POINTER 0
RP0 R232 (E8h) System Read/Write
Register Pointer 0
Reset Value : undefined
b7-b3 = RG7-RG3: Register Group number . These
bits contain the number (from 0 to 31) of the group
of working registers indicated in the instructions
srp0 or srp. When using a 16-register group, a
number between 0 and 31 must be used in the srp
instruction indicating one of the two adjacent 8-reg-
ister group of working registers used. RG7 is the
MSB.
b2 = RPS: Register Pointer Selector . This bit is set
by the instructions srp0 and srp1 to indicate that a
double register pointing mode is used. Otherwise,
the instruction srp resets the RPS bit to zero to indi-
cate that a single register pointing mode is used.
b1,b0 = D1,D0: These bits are fixed by hardware to
zero and are not affected by any writing instruction
trying to modify their value.
REGISTER POINTER 1
RP1 R233 (E9h) System Read/Write
Register Pointer 1
Reset Value : undefined
This register is used only with double register
pointing mode; otherwise, using single register
pointing mode, the RP1R register has to be consid-
ered as reserved and not usable as a general pur-
pose register.
b7-b3 = RG7-RG3: Register Group number . These
bits contain the number (from 0 to 31) of the group
of 8 working registers indicated in the instructions
srp1. Bit 7 is the MSB.
b2 = RPS: Register Pointer Selector . This bit is
automatically set by the instructions srp0 and
srp1 to indicate that a double register pointing
mode is used. Otherwise the instruction srp reset
the RPS bit to zero to indicate that a single register
pointing mode is used.
b1,b0 = D1,D0: These bits are hardware fixed to
zero and are not affected by any writing instruction
trying to modify their value.
Note. If working in twin 8-register group mode but
only using srp0 (i.e. only using one 8-register
group) the unused register (R233) is to be consid-
ered as reserved and not usable as a general pur-
pose register.
The group of registers immediately below the sys-
tem registers (i.e. group D, R208-R223) can only
be accessed via the Register Pointers. To address
group D then, it is necessary to set the Register
Pointer to group D and then use the addressing
procedure for working registers. The programmer
is required to remember that the group D should be
used as a stacking area. This point is also covered
in the Stack Pointers paragraph.
ST9291
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