COP8TAB9 Datasheet, PDF (48/59 Pages) National Semiconductor (TI) – 8-Bit CMOS Flash Microcontroller with 2k Byte or 4k Byte Memory

19.0 Instruction Set (Continued)
19.3.2 Tranfer-of-Control Addressing Modes
Program instructions are usually executed in sequential or-
der. However, Jump instructions can be used to change the
normal execution sequence. Several transfer-of-control ad-
dressing modes are available to specify jump addresses.
A change in program flow requires a non-incremental
change in the Program Counter contents. The Program
Counter consists of two bytes, designated the upper byte
(PCU) and lower byte (PCL). The most significant bit of PCU
is not used, leaving 15 bits to address the program memory.
Different addressing modes are used to specify the new
address for the Program Counter. The choice of addressing
mode depends primarily on the distance of the jump. Farther
jumps sometimes require more instruction bytes in order to
completely specify the new Program Counter contents.
The available transfer-of-control addressing modes are:
• Jump Relative
• Jump Absolute
• Jump Absolute Long
• Jump Indirect
The transfer-of-control addressing modes are described be-
low. Each description includes an example of a Jump in-
struction using a particular addressing mode, and the effect
on the Program Counter bytes of executing that instruction.
Jump Relative. In this 1-byte instruction, six bits of the
instruction opcode specify the distance of the jump from the
current program memory location. The distance of the jump
can range from −31 to +32. A JP+1 instruction is not allowed.
The programmer should use a NOP instead.
Example: Jump Relative
JP 0A
Reg
PCU
PCL
Contents
Before
02 Hex
05 Hex
Contents
After
02 Hex
0F Hex
Jump Absolute. In this 2-byte instruction, 12 bits of the
instruction opcode specify the new contents of the Program
Counter. The upper three bits of the Program Counter re-
main unchanged, restricting the new Program Counter ad-
dress to the same 4-kbyte address space as the current
instruction. (This restriction is relevant only in devices using
more than one 4-kbyte program memory space.)
Example: Jump Absolute
JMP 0125
Reg
PCU
PCL
Contents
Before
0C Hex
77 Hex
Contents
After
01 Hex
25 Hex
Jump Absolute Long. In this 3-byte instruction, 15 bits of
the instruction opcode specify the new contents of the Pro-
gram Counter.
Example: Jump Absolute Long
JMP 03625
Reg/
Memory
PCU
PCL
Contents
Before
42 Hex
36 Hex
Contents
After
36 Hex
25 Hex
Jump Indirect. In this 1-byte instruction, the lower byte of
the jump address is obtained from a table stored in program
memory, with the Accumulator serving as the low order byte
of a pointer into program memory. For purposes of access-
ing program memory, the contents of the Accumulator are
written to PCL (temporarily). The data pointed to by the
Program Counter (PCH/PCL) is loaded into PCL, while PCH
remains unchanged.
Example: Jump Indirect
JID
Reg/
Memory
PCU
PCL
Accumulator
Memory
Location
0126 Hex
Contents
Before
01 Hex
C4 Hex
26 Hex
Contents
After
01 Hex
32 Hex
26 Hex
32 Hex
32 Hex
The VIS instruction is a special case of the Indirect Transfer
of Control addressing mode, where the double-byte vector
associated with the interrupt is transferred from adjacent
addresses in program memory into the Program Counter in
order to jump to the associated interrupt service routine.
19.4 INSTRUCTION TYPES
The instruction set contains a wide variety of instructions.
The available instructions are listed below, organized into
related groups.
Some instructions test a condition and skip the next instruc-
tion if the condition is not true. Skipped instructions are
executed as no-operation (NOP) instructions.
19.4.1 Arithmetic Instructions
The arithmetic instructions perform binary arithmetic such as
addition and subtraction, with or without the Carry bit.
Add (ADD)
Add with Carry (ADC)
Subtract with Carry (SUBC)
Increment (INC)
Decrement (DEC)
Decimal Correct (DCOR)
Clear Accumulator (CLR)
Set Carry (SC)
Reset Carry (RC)
19.4.2 Transfer-of-Control Instructions
The transfer-of-control instructions change the usual se-
quential program flow by altering the contents of the Pro-
gram Counter. The Jump to Subroutine instructions save the
Program Counter contents on the stack before jumping; the
Return instructions pop the top of the stack back into the
Program Counter.
Jump Relative (JP)
Jump Absolute (JMP)
www.national.com
48