MAS281
Figure 10: Data Formats
Figure 9: Register Set Model
4.2.4 IMMEDIATE LONG (IM, IMX)
4.2.5.1 IMMEDIATE SHORT POSITIVE (ISP)
There are two formats which implement Immediate Long
Addressing; one allows indexing and one does not. For the
indexable form, if the specified index register, RX, is not equal
to zero, the contents of RX are added to the immediate field to
form the required operand; otherwise, the immediate field
contains the required operand.
4.2.5 IMMEDIATE SHORT (IS)
The immediate operand is treated as a positive integer
between 1 and 16.
4.2.5.2 IMMEDIATE SHORT NEGATIVE (ISN)
The immediate operand is treated as a negative integer
between -1 and -16. Its internal form is a two’s complement,
sign-extended 16-bit number.
In this mode the required 4-bit operand is contained within
the 16-bit instruction. The Immediate Short addressing mode
accommodates two formats; one which interprets the contents
of the immediate field as positive data and the other which
interprets the contents of the immediate field as a negative
data.
4.2.6 INSTRUCTION COUNTER RELATIVE (ICR)
This addressing mode is used for 16-bit branch
instructions. The contents of the instruction counter minus two
(the address of the current instruction) is added to the sign-
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