Nintendo DMG-01 Manual - Page 20

Warning, Examples

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2.7.3. Low-Power Mode Game BoyTM CPU Manual is executed. If interrupts are disabled (DI) then halt doesn't suspend operation but it does cause the program counter to stop counting for one instruction on the GB,GBP, and SGB as mentioned below. Depending on how much CPU time is required by a game, the HALT instruction can extend battery life anywhere from 5 to 50% or possibly more. WARNING: The instruction immediately following the HALT instruction is "skipped" when interrupts are disabled (DI) on the GB,GBP, and SGB. As a result, always put a NOP after the HALT instruction. This instruction skipping doesn't occur when interrupts are enabled (EI). This "skipping" does not seem to occur on the GameBoy Color even in regular GB mode. ($143=$00) EXAMPLES from Martin Korth who documented this problem: (assuming interrupts disabled for all examples) 1) This code causes the 'a' register to be incremented TWICE. 76 halt 3C inc a 2) The next example is a bit more difficult. The following code 76 halt FA 34 12 ld a,(1234) is effectively executed as 76 halt Page 20 V 1.01

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2.7.3. Low-Power Mode
Game Boy
TM
CPU Manual
is executed. If interrupts are disabled (DI) then
halt doesn't suspend operation but it does cause
the program counter to stop counting for one
instruction on the GB,GBP, and SGB as mentioned below.
Depending on how much CPU time is required by a game,
the HALT instruction can extend battery life anywhere
from 5 to 50% or possibly more.
WARNING:
The instruction immediately following the
HALT instruction is "skipped" when interrupts are
disabled (DI) on the GB,GBP, and SGB. As a result,
always put a NOP after the HALT instruction. This
instruction skipping doesn't occur when interrupts
are enabled (EI).
This "skipping" does not seem to occur on the
GameBoy Color even in regular GB mode. ($143=$00)
EXAMPLES
from Martin Korth who documented this
problem:
(assuming interrupts disabled for all examples)
1) This code causes the 'a' register to be
incremented TWICE.
76
halt
3C
inc
a
2) The next example is a bit more difficult.
The following code
76
halt
FA 34 12
ld
a,(1234)
is effectively executed as
76
halt
Page 20
V 1.01