ST9 datasheet(15/146 Pages) STMICROELECTRONICS

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ST9 USER GUIDE

by choosing to store frequently used data in registers, leaving less frequently used data in

memory.

3.1.2 Register File

The ST9 has a special addressing space for registers, providing 256 different register ad-

dresses. This large amount of registers gives you considerable flexibility in allocating vari-

ables. Register addresses are coded using one byte. You can use any of these registers to

hold data or as a pointer either to other registers or to bytes in memory(1). Contrast this with

processors that feature a certain number of registers, but in which some of these registers are

meant to be only pointers or indexes, and some others not. These processors only allow trans-

fers between memory and registers. The register organization of the ST9 gives you a real ad-

vantage you can make use of.

3.1.3 Direct access to the Register File

The entire Register File can be accessed directly by its address prefixed by an “R” except for

For example to address the register at the address 73, address it as R73, R49h or R1001001b

in decimal, hexadecimal or binary. For a double register (16 bits) you can use the “RR” prefix

to address any data with an even address.

Any register can be given a user-defined name.

In C language:

#pragma register_file data1

char data1;

#pragma register_file data2 60

int data2;

and accessed with:

data1=13;

data2=0x1234;

data1 is automatically allocated in the register file by the linker as no register number is pro-

vided. data2 is manually allocated in register RR60. This pragma should be repeated in each

file where the variable is made visible through an external declaration.

However using these registers needs an additional byte with the instruction mnemonic. Using

Working Registers is more efficient, because it avoids using this address byte.

(1) Except for group E (14) reserved for the system registers and group F (15) reserved for

the peripherals.

ST9 データシート(PDF) 15 Page - STMicroelectronics