Maxim Integrated MAXQ7667 - Manual (Page 32)

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MAXQ7667 User’s Guide
3.3.4 Moving Values Between Registers of Different Sizes
Before covering some transfer scenarios that might arise, a special register must be introduced that will be used 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.
3.3.4.1 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]
3.3.4.2 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.
move GR, LC[0] ; move LC[0] to the GR register
move IC, GRH ; copy the high byte into the IC register
3.3.4.3 16-Bit Destination ← Concatenation (8-Bit Source, 8-Bit Source)
Two 8-bit source registers can be concatenated and stored into a 16-bit destination by using the prefix register to hold the high-order
byte for the concatenated transfer. An additional cycle may be required if either source byte register index is greater than 0Fh or the
16-bit destination is greater than 07h.
move PFX[0], IC ; load high order source byte IC into PFX
move @DP[0], AP ; store @DP[0] the concatenation of IC:AP
; 16-bit destination sub-index: dst=08h
; 8-bit source sub-indexes:
; high=10h, low=11h
move PFX[1], #00h ;
move PFX[3], high ; PFX=00:high
move dst, low ; dst=high:low
3.3.4.4 Low (16-Bit Destination) ← 8-Bit Source
To modify only the low byte of a given 16-bit destination, the 16-bit register should be moved into the GR register such that the high
byte can be singulated and the low byte written exclusively. An additional cycle is required if the destination index is greater than 0Fh.
move GR, DP[0] ; move DP[0] to the GR register
move PFX[0], GRH ; get the high byte of DP[0] via GRH
move DP[0], #20h ; store the new DP[0] value
; 16-bit destination sub-index: dst=10h
; 8-bit source sub-index: src=11h
move PFX[1], #00h ;
move GR, dst ; read dst word to the GR register
move PFX[5], GRH ; get the high byte of dst via GRH
move dst, src ; store the new dst value