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ST9 USER GUIDE

5.4.3.2 Initialisation Code

The initialisation code mainly has to initialise the core. The core contains certain control regis-

ters that must be set to correct values or the program will fail to start. They are listed in the

table below. They are in the order they are initialized in a typical start-up file, though this order

is somewhat arbitrary.

Number

Name

Function(*)

Comments

235

MODER

Selects the space

where the stacks re-

side (register file or

memory), main clock

divider on/off, clock

prescaler division

rate.

If power and electromagnetic interference are not a

concern, the prescaler can be set to zero so that the

processor runs at full speed. Otherwise, the speed/

consumption trade-off can be handled at will accord-

ing to the needs of the program.

231

FLAGR

Selects the single

space of twin-space

mode for the memory.

In the “ST9” mode the DP bit of this register selects

program memory when cleared, or data memory

when set. It must be changed using the sdm and spm

instructions. Typically, one of these is used at the be-

ginning of the program and remains unchanged after-

wards. On some occasions, it may be changed, for

example to access tables of constants in ROM if two

spaces are used.

Therefore in “ST9” mode, DP must be set with the

sdm instruction (used only once in the start-up pro-

gram) to set the use of DPR register to address data

memory. The spm instruction must not be used. To

access data through the CSR register, use the ldpd,

lddp or ldpp instruction.

238-239

SSP

Position of the top of

the system stack in

memory or register

file

No subroutine or function call may be performed be-

fore the initialisation of SSP and MODER. Note that

since a call first decrements the stack pointer by two

and then writes the return address, if the stack is po-

sitioned in the register file, it may be initialised to the

space.

236-237

USP

Position of the top of

the user stack in

memory or register

file. Unused if only

one stack is used.

232-233

RP0-RP1

Selects the mode for

the working registers

as well as the abso-

lute register numbers

for r0 and r8.

These registers are set using the srp, srp0 and srp1

instructions. See the description of working registers

Section 3.1.4.

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