ST52X420 datasheet(17/84 Pages) STMICROELECTRONICS

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ST52T410/ST52T420/E420

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The ST52T410/ST52x420 core uses flags that

correspond to the actual mode. As soon as an

interrupt is generated the ST52T410/ST52x420

core uses the interrupt flags instead of the normal

flags.

Each interrupt level has its own set of flags, which

is saved in the STACK together with the Program

Counter. These flags are restored from the STACK

automatically when a RETI instruction is executed.

If the MCU was in normal mode before an interrupt,

the normal flags are restored when the RETI

instruction is executed.

Note: A CALL subroutine is a normal mode

execution. For this reason, a RET instruction,

consequent to a CALL instruction does not affect

the normal mode set of flags.

Flags are not cleared during context switching and

remain in the state they were at the end of the last

interrupt routine switching.

The Carry flag is set when an overflow occurs

during arithmetic operations, otherwise it

cleared.

The Sign flag is set when an underflow occurs

during

arithmetic operations,

otherwise

cleared.

2.3 Address Spaces

ST52T410/ST52x420 has four separate address

spaces:

Figure 2.3 Address Spaces Description

I RAM: 128 Bytes

I Input Registers: 18 8-bit registers

I Output Registers 9 8-bit registers

I Configuration Registers: 17 8-bit registers

I Program memory up to 4K Bytes

Program memory will be described in further

details in the MEMORY section

2.3.1 RAM and STACK.

RAM memory consists of 128 general purpose 8-

bit RAM registers.

All the registers in RAM can be specified by using

a decimal address. For example, 0 identifies the

first register of RAM.

To read or write RAM registers LOAD instructions

must be used. See Table 2.5

Each interrupt level has its own set of flags, which

is saved in the STACK together with the Program

Counter. These flags are restored from the STACK

automatically when a RETI instruction is executed.

When the instructions like Interrupt request or

CALL are executed, a STACK level is used to push

the PC.

The STACK is located in RAM. For each level of

stack, 2 bytes of RAM are used. The values of this

stack are stored from the last RAM register

(address 127). The maximum level of stack

must be less than 128.

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