Figure 4-1 Address Generation Unit Block Diagram; Figure 4-2 Address Generation Unit Programming Model - Motorola DSP56800 Manual

M01
All four address pointer registers and the SP are used in generating addresses in the register indirect
addressing modes. The offset register can be used by all four address pointer registers and the SP, whereas
the modulo register can be used by the R0 or by both the R0 and R1 pointer registers.
Whereas all the address pointer registers and the SP can be used in many addressing modes, there are some
instructions that only work with a specific address pointer register. These cases are presented in Table 4-5
on page 4-9.
The address generation unit is connected to four major buses: CGDB, XAB1, XAB2, and PAB. The
CGDB is used to read or write any of the address generation unit registers. The XAB1 and XAB2 provide
a primary and secondary address, respectively, to the X data memory, and the PAB provides the address
when accessing the program memory.
A block diagram of the address generation unit is shown in Figure 4-1, and its corresponding programming
model is shown in Figure 4-2. The blocks and registers are explained in the following subsections.
15
Pointer
Registers
Figure 4-2. Address Generation Unit Programming Model
CGDB(15:0)
N
Modulo
Arithmetic
Unit
Figure 4-1. Address Generation Unit Block Diagram
0 15
R0
R1
R2
R3
SP
Address Generation Unit
Architecture and Programming Model
SP
R0
R1
R2
R3
PAB(15:0) XAB1(15:0)
0 15
N
Offset
Register
Inc./Dec.
R3 Only
XAB2(15:0)
AA0014
0
M01
Modifier
Register
AA0015
4-3