ST72F324BK6TAS STMicroelectronics, ST72F324BK6TAS Datasheet - Page 142

• Manufacturer Part Number: ST72F324BK6TAS
• Description: 8-BIT MCU
• Manufacturer: STMicroelectronics

Instruction set
142/198
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
PC-1
PC
PC+1
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
PIX 92
PIY 91
Replace an X based instruction using immediate, direct, indexed, or inherent
End of previous instruction
Prebyte
Opcode
effective address
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.
Replace an instruction using X indirect indexed addressing mode by a Y one.
Additional word (0 to 2) according to the number of bytes required to compute the
addressing mode by a Y one.
Doc ID13466 Rev 4
ST72324B-Auto