ST9291 datasheet(13/20 Pages) STMICROELECTRONICS

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1.1.3.3 Register Pointing Techniques

Two registers, R232 and R233, within the system

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

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.

RP0 R232 (E8h) System Read/Write

Reset Value : undefined

RG7 RG6 RG5 RG4 RG3 RPS

b7-b3 = RG7-RG3:

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:

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.

RP1 R233 (E9h) System Read/Write

Reset Value : undefined

RG7 RG6 RG5 RG4 RG3 RPS

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:

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:

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.

SYSTEM REGISTERS (Continued)

ST9291

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