Maxim Integrated DS4830A Optical Microcontroller - Manual (Page 190)

DS4830A User’s Guide
190
SECTION 23 – PROGRAMMING
The following section provides a programming overview of the DS4830A. For full details on the instruction set, as
well as System Register and Peripheral Register detailed bit descriptions, see the appropriate sections in this
user’s guide.
23.1 – Addressing Modes
The instruction set for the DS4830A provides three different addressing modes: direct, indirect and immediate.
The direct addressing mode can be used to specify either source or destination registers, such as:
move A[0], A[1]
; copy accumulator 1 to accumulator 0
push A[0] ; push accumulator 0 on the stack
add A[1] ; add accumulator 1 to the active accumulator
Direct addressing is also used to specify addressable bits within registers.
move C, Acc.0 ; copy bit zero of the active accumulator to the carry flag
move PO0.3, #1 ; set bit three of port 0 Output register
Indirect addressing, in which a register contains a source or destination address, is used only in a few cases.
move @DP[0], A[0] ; copy accumulator 0 to the data memory location pointed to by data pointer 0
move A[0], @SP-- ; where @SP-- is used to pop the data pointed to by the stack pointer register
Immediate addressing is used to provide values to be directly loaded into registers or used as operands.
move A[0], #10h ; set accumulator 1 to 10h/16d
23.2 – Prefixing Operations
All instructions on the DS4830A are 16 bits long and execute in a single cycle. However, some operations require
more data than can be specified in a single cycle or require that high order register index bits be set to achieve the
desired transfer. In these cases, the prefix register module PFX is loaded with temporary data and/or required
register index bits to be used by the following instruction. The PFX module only holds loaded data for a single cycle
before it clears to zero.
Instruction prefixing is required for the following operations, which effectively makes them two-cycle operations.
• When providing a 16-bit immediate value for an operation (e.g. loading a 16-bit register, ALU operation,
supplying an absolute program branch destination), the PFX module must be loaded in the previous cycle with
the high byte of the 16-bit immediate value unless that high byte is zero. One exception to this rule is when
supplying an absolute branch destination to 0023h. In this case, PFX still must be written with 00h. Otherwise,
the branch instruction would be considered a relative one instead of the desired absolute branch.
• When selecting registers with indexes greater than 07h within a module as destinations for a transfer or registers
with indexes greater than 0Fh within a module as sources, the PFX[n] register must be loaded in the previous
cycle. This can be combined with the previous item.
Generally, prefixing operations can be inserted automatically by the assembler as needed, so that (for example)
move DP[0], #1234h
actually assembles as
move PFX[0], #12h
move DP[0], #34h
However, the operation
move DP[0], #0055h
does not require a prefixing operation even though the register DP[0] is 16-bit. This is because the prefix value
defaults to zero, so the line
move PFX[0], #00h is not required.