Hardware Reference
In-Depth Information
the designers of the 8088 did not include a Web browser in their test set. Neverthe-
less, quite a few of that product's descendants are now used primarily for Web
surfing, so the decisions made 20 years ago may be completely wrong for current
applications. However, in the name of backward compatibility, once a feature gets
in there, it is impossible to get it out.
EBP
i
in EAX
Other
local
variables
SIB Mode references
M[4 * EAX + EBP + 8]
Stack
frame
a [0]
EBP+8
a [1]
EBP+12
a [2]
EBP+16
Figure 5-27.
Access to
a
[
i
].
In the OMAP4430, all instructions use immediate or register addressing except
those that address memory. For register mode, the 5 bits simply tell which register
to use. For immediate mode, an (unsigned) 12-bit constant provides the data. No
other modes are present for the arithmetic, logical, and similar instructions.
Two kinds of instructions address memory: loads (
LDR
) and stores (
STR
).
LDR
and
STR
instructions have three modes for addressing memory. The first mode
computes the sum of two registers and then indirects through it. The second mode
computes the address as the sum of a base register and a 13-bit signed offset. The
third addressing mode computes an address equal to the program counter (PC) plus
a 13-bit signed offset. This third addressing mode, called PC-relative addressing, is
useful for loading program constants which are stored with the program's code.
The ATmega168 has a fairly regular addressing structure. There are four basic
modes. The first is register mode, in which the operand is in a register. Registers
can be used as both sources and destinations. The second is immediate mode,
where an 8-bit unsigned immediate value can be encoded into an instruction.
