Samsung KS57C2308 Manual page 77

KS57C2308/P2308/C2316/P2316
Reducing Instruction Redundancy
When redundant instructions such as LD A,#im and LD EA,#imm are used consecutively in a program sequence,
only the first instruction is executed, but the following redundant instructions are ignored, that is, they are handled
like a NOP instruction. When LD HL,#imm instructions are used consecutively, the following redundant
instructions are also ignored.
In the following example, only the "LD A, #im" instruction will be executed. The 8-bit load instruction which
follows it is interpreted as redundant and is ignored:
LD A,#im
LD EA,#imm
In this example, the statements "LD A,#2H" and "LD A,#3H" are ignored:
BITR EMB
LD A,#1H
LD A,#2H
LD A,#3H
LD 23H,A
If consecutive LD HL, #imm instructions (load 8-bit immediate data to the 8-bit memory pointer pair, HL) are
detected, only the first LD is executed and the LDs which immediately follow are ignored. For example,
LD HL,#10H
LD HL,#20H
LD A,#3H
LD EA,#35H
LD @HL,A
If an instruction reference with a REF instruction has a redundancy effect, the following conditions apply:
— If the instruction preceding the REF has a redundancy effect, this effect is cancelled and the referenced
instruction is not skipped.
— If the instruction following the REF has a redundancy effect, the instruction following the REF is skipped.
+ +
PROGRAMMING TIP — Example of the Instruction Redundancy Effect
ORG
ABC LD
ORG
•
•
•
LD
REF
•
•
•
REF
LD
; Load 4-bit immediate data (#im) to accumulator
; Load 8-bit immediate data (#imm) to extended accumulator
; Execute instruction
; Ignore, redundant instruction
; Ignore, redundant instruction
; Execute instruction, 023H
; HL 10H
; Ignore, redundant instruction
; A 3H
; Ignore, redundant instruction
; (10H) 3H
0020H
EA,#30H
0080H
EA,#40H
ABC
ABC
EA,#50H
#1H
; Stored in REF instruction reference area
; Redundancy effect is encountered
; No skip (EA #30H)
; EA #30H
; Skip
SAM47 INSTRUCTION SET
5-3