Commodore Plus 4 - Page 261

The 6502 Microprocessor 249
. 2002 A9 01 LDA #$01 Start looking at bit 0.
. 2004 2C 00 21 BIT $2100 See if this bit is set in $2100.
. 2007 FO 01 BEQ $200A If not, go on to next bit.
. 2009 E8 INX Increment the bit count.
. 200A OA ASL Shift left to the next bit.
. 200B 90 P7 BCC $2004 When carry is set, then program is
done.
. 200D 00 BRK Stop execution.
The number of bits set to 1 in $2100 is in the X register when the BRK
instruction is executed. The contents of the registers are displayed when a BRK is
executed by the BRK instruction processor.
BMI—Branch. If Negative Flag Is Set
This instruction examines the current status of the negative flag bit. If it is set to 1,
the branch occurs. If it is 0, the branch does not occur, and execution continues
with the following instruction.
Operation: Branch on N - 1 Flags Affected: None
Addressing Mode Syntax Opcode Bytes Cycles
Relative BMI $hhhh 30 2_ 2*
* Add 1 cycle when the branch is taken. Add 2 cycles when it is taken across a page boundary.
Example: In this program, the locations from $2100 to $217F that hold a nonnegative
number are counted. The BMI instruction is used to determine when a negative
number is encountered (it is not counted).
.Y counts the nonnegative numbers.
.X contains the pointer to the current
location.
Get value of next location.
If it is negative, do not count it.
Count a nonnegative location.
Bump the pointer.
If not finished, keep going.
Stop processing.
The count is displayed as the contents of the Y register when the BRK
instruction is executed.
. 2000
. 2002
. 2004
. 2007
. 2009
. 200A
. 200B
. 200D
AO
A2
BD
30
08
E8
DO
00
00
80
80
01
P7
LDY
LDX
20 LDA
BMI
IEY
INX
BNE
BRK
#$oo
#$80
$2080,
$200A
$2004