Maxim Integrated DS4830A Optical Microcontroller - Manual (Page 191)

DS4830A User’s Guide
191
23.3 – Reading and Writing Registers
All functions in the DS4830A are accessed through registers, either directly or indirectly. This section discusses
loading registers with immediate values and transferring values between registers of the same size and different
sizes.
23.3.1 – Loading an 8-Bit Register with an Immediate Value
Any writeable 8-bit register with a sub-index from 0h to 7h within its module can be loaded with an immediate value
in a single cycle using the MOVE instruction.
move AP, #05h ; load accumulator pointer register with 5 hex
Writeable 8-bit registers with sub-indexes 8h and higher can be loaded with an immediate value using MOVE as
well, but an additional cycle is required to set the prefix value for the destination.
move WDCN, #33h ; assembles to: move PFX[2], #00h
; move (WDCN-80h), #33h
23.3.2 – Loading a 16-Bit Register with a 16-Bit Immediate Value
Any writeable 16-bit register with a sub-index from 0h to 07h can be loaded with an immediate value in a single cycle
if the high byte of that immediate value is zero.
move LC[0], #0010h ; prefix defaults to zero for high byte
If the high byte of that immediate value is not zero or if the 16-bit destination sub-index is greater than 7h, an extra
cycle is required to load the prefix value for the high byte and/or the high order register index bits.
; high byte <> #00h
move LC[0], #0110h ; assembles to: move PFX[0], #01h
; move LC[0], #10h
; destination sub-index > 7h
move A[8], #0034h ; assembles to: move PFX[2], #00h
; move (A[8]-80h), #34h
23.3.3 – Moving Values Between Registers of the Same Size
Moving data between same-size registers can be done in a single-cycle MOVE if the destination register’s index is
from 0h to 7h and the source register index is between 0h and Fh.
move A[0], A[8] ; copy accumulator 8 to accumulator 0
move LC[0], LC[1] ; copy loop counter 1 to loop counter 0
If the destination register’s index is greater than 7h or if the source register index is greater than Fh, prefixing is
required.
move A[15], A[0] ; assembles to: move PFX[2], #00h
; move (A[15]-80h), A[0]
23.3.4 – Moving Values Between Registers of Different Sizes
Before covering some transfer scenarios that might arise, a special register must be introduced that will be utilized in
many of these cases. The 16-bit General Register (GR) is expressly provided for performing byte singulation of 16-
bit words. The high and low bytes of GR are individually accessible in the GRH and GRL registers respectively. A
read-only GRS register makes a byte-swapped version of GR accessible and the GRXL register provides a sign-
extended version of GRL.
8-Bit Destination
 Low Byte (16-Bit Source)
The simplest transfer possibility would be loading an 8-bit register with the low byte of a 16-bit register. This transfer
does not require use of GR and requires a prefix only if the destination or source register are outside of the single
cycle write or read regions, 0-7h and 0-Fh, respectively.
move OFFS, LC[0] ; copy the low byte of LC[0] to the OFFS register
move IMR, @DP[1] ; copy the low byte @DP[1] to the IMR register
move WDCN, LC[0] ; assembles to: move PFX[2], #00h
; move (WDCON-80h), LC[0]
8-Bit Destination  High Byte (16-Bit Source)
If, however, we needed to load an 8-bit register with the high byte of a 16-bit source, it would be best to use the GR
register. Transferring the 16-bit source to the GR register adds a single cycle.