ST7285C datasheet(78/117 Pages) STMICROELECTRONICS

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ST7285C

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 internal

registers include 2 index registers, an accumula-

tor, a 16-bit Program Counter, a stack pointer and

a condition code register. The two Index registers

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 or-

der 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 subdi-

vided 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 page zero

(0000 - 00FFh range), but the instruction size is

more compact, and faster. All memory to memo-

ry instructions are only working with short ad-

dressing 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 does not 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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