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3-5

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

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 gr

eater than 0Fh or the

16-bit destination is greater than 07h.

move PFX[0], IC ; load high order source byte IC into PFX

move @++SP, 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

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

High (16-bit destination) ← 8-bit source

To modify only the high byte of a given 16-bit destination, the 16-bit register should be moved into the GR r

egister such that the low

byte can be singulated and the high byte can be written exclusively. Additional cycles are requir

ed if the destination index is greater

than 0Fh or if the source index is greater than 0Fh.

move GR, DP[0] ; move DP[0] to the GR register

move PFX[0], #20h ; get the high byte of DP[0] via GRH

move DP[0], GRL ; 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[1], #00h

move PFX[4], src ; get the new src byte

move dst, GRL ; store the new dst value

If the high byte needs to be clear

ed to 00h, the operation can be shortened by transferring only the GRL byte to the 16-bit destination

(example follows):

move GR, DP[0] ; move DP[0] to the GR register

move DP[0], GRL ; store the new DP[0] value, 00h used for high byte

MAXQ Family User’s Guide

Maxim Integrated