Commodore Plus 4 - Page 292

280 Machine Language on the Commodore Plus/4
The carry flag is used to transmit the borrow information between the bytes.
Before the first SBC, the carry is set, so the first result is correct. If a borrow is
produced (i.e., the carry is cleared), it is automatically accounted for in the next
byte because the program does not set the carry before performing the next SBC.
SEC—Set the Carry Flag
The carry flag is set to 1.
Operation: 1 — C Flags Affected: C
Addressing Mode Syntax Opcode Bytes Cycles
Implied SEC 38 1 2
Example: This program subtracts the contents of $2101 from the contents of $2100 and
stores the result in $2102. Since the SBC instruction adds the value of the carry to
the sum of the accumulator and the one's complement of the operand, the SEC
instruction is used to set the carry flag prior to the subtraction. This ensures that
the result is exactly the first value minus the second value.
2000
2001
2004
2007
200A
38
AD
ED
8D
00
00
01
02
21
21
21
SEC
LDA
SBC
STA
BRK
12100
12101
$2102
Set the carry flag to
subtract.
Get the first value.
Subtract the second
Store the result.
Stop processing.
prepare to
value.
Use the M (Memory) command to examine and change the contents of
$2100-$2102.
SED—Set Decimal Mode
The processor is put into decimal mode.
Operation: 1 — D Flags Affected: D
Addressing Mode Syntax Opcode Bytes Cycles
Implied SED F8 1 2
See the section on decimal mode.