Commodore Plus 4 - Page 259

The 6802 Microprocessor 847
Use the M (Memory) command to examine and modify the contents of
$2100-52102 before executing the program (using G 2000). Then check the
contents of $2100-$2102 after the program executes. The values in $2101-$2102
are the sum of their earlier values and the value in $2100.
BCS—Branch If Carry Flag Is Set
This instruction examines the current status of the carry 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 C- 1 Flags Affected: None
Addressing Mode Syntax Opcode Bytes Cycles
Relative BCS $hhhh B0 2 2*
* Add 1 cycle when the branch is taken. Add 2 cycles when it is taken across a page boundary.
Example: When subtracting a 1-byte number from a 2-byte number, you can use standard
double precision subtraction, but the operation may be speeded up by using a
BCS to determine when to decrement the high byte. In this example, the 8-bit
number stored at $2100 is subtracted from the 16-bit number stored at
$2101-$2102 (high byte to low byte), and the result is stored in $2101-$2102.
2000
2001
2004
2007
200A
200C
200F
38
AD
ED
8D
BO
CE
00
OS
00
02
03
01
21
21
21
21
SEC
LDA
SBC
STA
BCS
DEC
BRK
$2102
#2100
12102
$200F
12101
Set carry flag prior to subtraction.
Get low byte of $2101-$2102.
Subtract single byte.
Store the result.
If carry set, branch to finish.
Otherwise, decrement the high byte.
Stop execution.
Use the M (Memory) command to examine and modify the contents of
$2100-$2102 before executing the program (using G 2000). Then check the
contents of $2100-$2102 after the program executes. The values in $2101-$2102
are the difference between their earlier values and the value in $2100.
BEQ—Branch If Zero Mag Is Set
This instruction examines the current status of the zero 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.