ST7LITE20F2 datasheet(115/166 Pages) STMICROELECTRONICS

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Instruction set

Doc ID 8349 Rev 5

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Using a prebyte

The instructions are described with 1 to 4 bytes.

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

●

PC: Opcode

●

PC+1: Additional word (0 to 2) according to the number of bytes required to compute

the effective address.

These prebytes enable 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.

12.2.1

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.

Conditional Branch

JRxx

−−

−

−−

−

Interruption management

TRAP

WFI

HALT

IRET

−−

−

Condition Code Flag modification

SIM

RIM

SCF

RCF

−−

−

Table 55.

Instruction groups (continued)

Group

Instructions

Table 56.

Instruction set overview

Mnemo

Description

Function/example

Dst

Src

ADC

Add with Carry

A = A + M + C

−

NZC

ADD

Addition

A = A + M

−

NZC

AND

Logical And

A = A . M

−−

−

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