Zilog Z16F2810 - Manual (Page 45)

UM018809-0611 Operand Addressing
ZNEO
®
CPU Core
User Manual
29
Direct Memory Addressing
A Direct Memory operand specifies a memory address to be used by the instruction.
Example. The following assembly language statement loads ALU register R7 with the
value in memory address 0000_B002H :
LD.SB R7, B002H
Any data operand which does not contain an immediate value (#n) or register name (Rn) is
assumed to be a memory address. Depending on the instruction, a direct memory address
can be used in either the source or destination operand, but a destination’s effective
address must be a writable memory or I/O location.
ZNEO CPU uses 32-bit memory addresses, but it includes instruction opcodes which
accept 16-bit addresses. A 16-bit address operand in object code is sign-extended by the
CPU (see the
Resizing Data section on page 31) to create the effective address used. This
feature splits the 16-bit address range between the highest and lowest 32K blocks of the 16
GB address space. Table 13 provides the 16-bit address ranges for object code.
Effective addresses are expressed as 32-bit values. Current devices ignore address bits
[31:24], providing a 24-bit address space.
Internal RAM and I/O memory falls in the range FFFF_8000H to FFFF_FFFFH
( FF_8000H to FF_FFFFH on devices that ignore address bits [31:24]), so 16-bit address-
ing provides efficient access to internal RAM and I/O memory.
The ZNEO CPU assembler does not automatically use 16-bit addressing if an unmodified
16-bit address is specified, as in the previous example. In this case the assembler selects
16-bit or 32-bit addressing to ensure the address is used as specified.
However, you can append address range mnemonics to specify whether the assembler
should use 16-bit or 32-bit addressing. The RAM, IODATA, and ROM mnemonics tell the
assembler to use 16-bit addressing, as shown in the following example statements:
LD.SB R7, B002H:RAM ; Effective address is FFFF_B002H
LD.SB R7, E002H:IODATA ; Effective address is FFFF_E002H
LD.SB R7, 3002H:ROM ; Effective address is 0000_3002H
Table 13. 16-Bit Addressing (Object Code Only)
16-Bit Address Range 32-Bit Effective Addresses Memory Space
0000H to 7FFFH 0000_000H to 0000_7FFFH ROM
8000H to FFFFH FFFF_8000H to FFFF_FFFFH RAM and I/O
Note: