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4 Using ST20-C1 instructions

4.2

Loading and storing

The loading and storing instructions are listed in Table 4.3.

On the ST20, the term

loading means pushing a value onto the evaluation stack. The

value to be loaded may be a value read from memory, a constant, a copy of another

Storing means popping a value from the evaluation

stack. The value may be written into memory or written into another register. The eval-

uation stack is described in section 3.3, and evaluation of expressions is described in

section 4.3.

Relative addresses are used for accessing memory in order to reduce code size, as

the operand values are smaller than full machine addresses. Data structures are

word-aligned, so relative addresses can be word offsets, reducing the operand size

further.

The most common operations performed by a program are loading and storing of a

small number of variables, and loading small literal values.

4.2.1

Loading constants

One primary instruction

ldc is provided for loading a general constant, for initializing a

variable or register or for a constant in an expression.

4.2.2

Local and non-local variables

When loading from and storing to memory, the ST20 distinguishes between local and

non-local addressing. Local addressing means that the address is given as a word

offset from the Wptr. Non-local addressing means that the address is given as a word

offset from the Areg. In practice, the Wptr points to the stack, so local addressing is

Mnemonic

Name

Description

ldc n

load constant

Load the constant

ldl n

load local

Load the value from

n words above Wptr.

stl n

store local

Store a value to

n words above Wptr.

ldnl n

load non-local

Load the value from

n words above Areg.

stnl n

store non-local

Store a value to

n words above Areg.

lbinc

load byte and increment

Load a byte and increment the address by 1 byte.

sbinc

store byte and increment

Store a byte and increment the address by 1 byte.

lsinc

load sixteen and increment

Load a half word and increment the address by 2 bytes.

lsxinc

load sixteen sign extended

and increment

Load a half word and sign extend to 32 bits and increment

the address by 2 bytes.

ssinc

store sixteen and increment

Store a half word and increment the address by 2 bytes.

lwinc

load word and increment

Load a word and increment the address by 4 bytes.

swinc

store word and increment

Store a word and increment the address by 4 bytes.

Table 4.3 Loading and storing instructions

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