ST20C2 datasheet(9/212 Pages) STMICROELECTRONICS

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Subscripts are used to indicate particular bits in a word; e.g. Aregi for bit i of Areg; and

Areg0..7 for the least significant byte of Areg. Note that bit 0 is the least significant bit in

a word, and bit 31 is the most significant bit.

Generally, if the description does not mention the state of a register or memory

location after the instruction, then the value will not be changed by the instruction.

One exception to this general rule is Iptr, which is assigned the address of the next

instruction in the code

before every instruction execution starts. The Iptr is included in

the description only when it is

directly affected by the instruction (e.g. in the jump

instruction). In these cases the address of the next instruction is indicated by the

comment “

next instruction”.

Scheduling operations

Some registers, such as the timer and scheduling list pointers and some special

workspace locations, can be changed at any time by scheduling operations. Changes

to these are included in the description only when they are

directly caused by the

instruction, and not just as an effect of any scheduling operation which might take

place.

1.7.3

Undefined v alues

Many instructions leave the contents of a register or memory location in an undefined

state. This means that the value of the location may be changed by the instruction, but

the new value cannot be easily defined, or is not a meaningful result of the instruction.

For example, when the integer stack is popped, Creg becomes undefined, i.e. it does

not contain any meaningful data. An undefined value is represented by the name

undefined. The values of registers which become undefined as a result of executing

an instruction are implementation dependent and are not guaranteed to be the same

on different members of the ST20 family of processors.

1.7.4

Data types

The instruction set includes operations on four sizes of data: 8, 16, 32 and 64-bit

objects. 8-bit and 16-bit data can represent signed or unsigned integers; 32-bit data

can represent addresses, signed or unsigned integers, or single length floating point

numbers; and 64-bit data can represent signed or unsigned integers, or double length

floating point v alues. Normally it is clear from the context (e.g. from the operators

used) whether a particular object represents a signed, unsigned or floating point

number. A subscripted label is added (e.g. Aregunsigned) to clarify where necessary.

1.7.5

Representing memory

The memory is represented by arrays of each data type. These are indexed by a value

representing a byte address. Access to the four data types is represented in the

instruction descriptions in the following way:

byte[

address]references a byte in memory at the given address

sixteen[

address]references a 16-bit object in memory

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