ST7DALIF2 datasheet(124/171 Pages) STMICROELECTRONICS

124 / 171 page

Instruction set

ST7DALIF2

124/171

Using a prebyte

The instructions are described with one to four opcodes.

In order to extend the number of available opcodes for an 8-bit CPU (256 opcodes), three

different prebyte opcodes are defined. These prebytes modify the meaning of the instruction

they precede.

The whole instruction becomes:

PC-2

End of previous instruction

PC-1

Prebyte

Opcode

PC+1

Additional word (0 to 2) according to the number of bytes required to compute the

effective address

These prebytes enable the instruction in Y as well as indirect addressing modes to be

implemented. They precede the opcode of the instruction in X or the instruction using direct

addressing mode. The prebytes are:

PDY 90

Replace an X based instruction using immediate, direct, indexed, or inherent

addressing mode by a Y one.

PIX 92

Replace an instruction using direct, direct bit, or direct relative addressing mode

to an instruction using the corresponding indirect addressing mode.

It also changes an instruction using X indexed addressing mode to an instruction

using indirect X indexed addressing mode.

PIY 91

Replace an instruction using X indirect indexed addressing mode by a Y one.

Illegal Opcode Reset

In order to provide enhanced robustness to the device against unexpected behavior, a

system of illegal opcode detection is implemented. If a code to be executed does not

correspond to any opcode or prebyte value, a reset is generated. This, combined with the

Watchdog, allows the detection and recovery from an unexpected fault or interference.

Note:

A valid prebyte associated with a valid opcode forming an unauthorized combination does

not generate a reset.

ST7DALIF2 Datasheet(PDF) 124 Page - STMicroelectronics