UM0144 datasheet(11/92 Pages) STMICROELECTRONICS

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ST7 Assembler Linker

2 ST7 Addressing Modes

UM0144

11/92

2.7

Indexed modes

The ST7 hardware supports four types of indexed mode:

●

indexed without offset,

●

indexed with a 8-bit unsigned offset (range [0 ,255]),

●

indexed with a 16-bit offset,

The source coding syntax is:

(X)

or (Y) for no-offset indexing.

(offset,X)

or (offset,Y)for indexed with offset.

Some instructions (such as ld A or add) support the first three types of indexed mode. Some

ST7 instructions (such as inc) only support the first two types (i.e. indexed without offset and

indexed with 8-bit unsigned offset). Examples:

2.8

Relative mode

This addressing mode is used to modify the Program Counter (PC) register value by adding an

8-bit signed offset to it (i.e. in the range -128 to +127). The relative addressing mode is made

up of two sub-modes:

●

relative (direct)—where the offset is following the op-code.

●

relative (indirect)—where the offset is defined in memory, whose address follows the op-

code.

Relative mode is used by the instructions JRxx, CALLR, and BTJx.

At source coding level, the target label is specified (and the assembler computes the

displacement).

2.9

High, low addressing modes

In some instances, it may be necessary to access the highest part of an address (8 highest

bits) or the lowest part of an address (8 lowest bits) as well. For this feature, the syntax is the

following one:

where expression is:

symbol.H (highest part) ,

symbol.L (lowest part).

A,(X)

; no-offset mode

A,(0,X)

; 8-bit offset mode

A,(127,X)

; 8-bit offset mode

A,(259,X)

; 16-bit offset mode

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