MOTOROLA
MC68349 USER'S MANUAL
3- 3
3.1.2 Function Code Signals
FC3–FC0 are outputs that indicate one of 16 address spaces to which the address
applies. Fifteen of these spaces are designated as either user or supervisor, program or
data, and normal or direct memory access (DMA) spaces. One other address space is
designated as CPU space to allow the CPU32+ to acquire specific control information not
normally associated with read or write bus cycles. FC3–FC0 are valid while AS is
asserted.
Function codes (see Table 3-1) can be considered as extensions of the 32-bit address that
can provide up to 16 different 4-Gbyte address spaces. Function codes are automatically
generated by the CPU32+ to select address spaces for data and program at both user and
supervisor privilege levels, a CPU address space for processor functions, and an alternate
master address space. User programs access only their own program and data areas to
increase protection of system integrity and can be restricted from accessing other
information. The S-bit in the CPU32+ status register is set for supervisor accesses and
cleared for user accesses to provide differentiation. Refer to 3.4 CPU Space Cycles for
more information.
Table 3-1. Address Space Encoding
Function Code Bits
3210
Address Spaces
0000
Reserved (Motorola)
0001
User Data Space
0010
User Program Space
0011
Reserved (User )
0100
Reserved (Motorola)
0101
Supervisor Data Space
0110
Supervisor Program Space
0111
CPU Space
1
x
DMA Space
3.1.3 Address Bus (A31–A0)
These signals are outputs that define the address of the byte (or the most significant byte)
to be transferred during a bus cycle. The MC68349 places the address on the bus at the
beginning of a bus cycle. The address is valid while AS is asserted.
3.1.4 Address Strobe (
AS)
This output timing signal indicates the validity of many control signals and the address on
the address bus. AS is asserted approximately one-half clock cycle after the beginning of
a bus cycle.