ST7294 datasheet(31/69 Pages) STMICROELECTRONICS

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ST7294

5 SOFTWARE

5.1 ST7 ARCHITECTURE

The 8-bit ST7 Core is designed for high code effi-

ciency. It contains 6 internal registers, 17 main ad-

dressing modes and 63 instructions. The 6 inter-

nal registers include 2 index registers, an accumu-

lator, a 16-bit Program Counter, a stack pointer

and a condition code register. The two Index reg-

isters X and Y enable Indexed Addressing modes

with or without offset, along with read-modify-write

type data manipulations. These registers simplify

branching routines and data modifications.

The 16-bit Program Counter is able to address up

to 64K of ROM/EPROM memory. The 6-bit Stack

Pointer provides access to a 64-level Stack and

an upgrade to an 8-bit Stack Pointer is foreseen in

order to be able to manage a 256-level Stack. The

Core also includes a Condition Code Register pro-

viding 5 Condition Flags that indicate the result of

the last instruction executed.

The 17 main Addressing modes, including Indirect

Relative and Indexed addressing, allow sophisti-

cated branching routines or CASE-type functions.

The Indexed Indirect Addressing mode, for in-

stance, permits look-up tables to be located any-

where in the address space, thus enabling very

flexible programming and compact C-based code.

The 63-instruction Instruction Set is 8-bit oriented

with a 2-byte average instruction size. This In-

struction Set offers, in addition to standard data

movement and logic/arithmetic functions, byte

multiplication, bit manipulation, data transfer be-

tween Stack and Accumulator (Push/Pop) with di-

rect stack access, as well as data transfer using

the X and Y registers.

5.2 ST7 ADDRESSING MODES

The ST7 Core features 17 different addressing

modes which can be classified in 7 main groups:

The ST7 Instruction set is designed to minimize

the number of bytes required per instruction: To

do so, most of the addressing modes may be sub-

divided in two sub-modes called long and short:

– The long addressing mode is the most powerful

because it can reach any byte in the 64kb ad-

dressing space, but the instruction is bigger and

slower than the short addressing mode.

– The short addressing mode is less powerful be-

cause it can generally only access the page zero

(00..FF range), but the instruction size is more

compact, and faster. All memory to memory in-

structions are only working with short addressing

modes (CLR, CPL, NEG, BSET, BRES, BTJT,

BTJF, INC, DEC, RLC, RRC, SLL, SRL, SRA,

SWAP)

Both modes have pros and cons, but the program-

mer doesn’t need to choose which one is the best:

the ST7 Assembler will always choose the best one.

Addressing Mode

Example

Inherent

nop

Immediate

ld A,#$55

Direct

ld A,$55

Indexed

ld A,($55,X)

Indirect

ld A,([$55],X)

Relative

jrne loop

Bit operation

bset

byte,#5

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