INTERNAL SPECIFICATIONS

4.3 lnternal Data Memory (RAM) Operating Procedures

4.3.1 Internal data memory indirect addressing

Operation of the internal data memory indirect increment instruction is described here as an example. This instruction (INC @Rr) is a 1-byte 1-machine cycle instruction (see Figure 4- 4). The indirect address register is specified by instruction code bit 0 data r where r denotes either register 0 or 1 in the register group specified by PSW RS0 and RS1 bank data. Register 0 is specified when the r data is 0, and register 1 is specified when the data is 1.

When this instruction is executed, register data is read from the specified register 0 or 1, and the read out register data is written into the data pointer for the data memory.

The data memory contents specified by the data pointer are read by the CPU into a temporary register. Then a subsequent increment (+1) by the ALU is followed by a return to the data memory at the address where the data were read out. In this way, the contents of the data memory at the address specified by the contents of R0 or R1 are incremented.

INC @Rr:

Instruction (OP)

Register

 

code portion

designation portion

 

 

 

 

Byte 1

0 0 0 0 0 1

1 r

 

 

 

 

 

7 6 5 4 3 2 1 0

Figure 4-4 INC @Rr bit arrangement

61

Page 68
Image 68
Sonic Alert msm85c154hvs Lnternal Data Memory RAM Operating Procedures, Internal data memory indirect addressing