MC68HC11 Instruction Set
Overview
• 68HC11 instruction set
o A quick look at the programming procedure
o The programmer's model
o Instruction types and formats
o Addressing modes
o A tour of the instruction set
• Readings for instruction set:
o Spasov, Chap. 2 and Appendix A
o HC11 Reference Manual, Appendix A
o HC11 Programming Reference Guide
Looking ahead at the
programming procedure:
• Soon you'll write programs in assembly language for
the HC11
• Here's a super simple example program:
; Super simple test program
ORG $B600
SEGMENT
code
TLOOP:INCA
INCB
JMP TLOOP
• Before the HC11 can run this program,
o The assembly-language code needs to be converted to machine
language, and the machine-language code needs to be
transferred to the HC11's memory
Cont..
Looking ahead at the
programming procedure:
• Here is the machine code that results from the
assembly-lang. program given above
Address
Contents
$B600
01001100
$B601
01011100
$B602
01111110
$B603
10110110
$B604
00000000
Cont.
Looking ahead at the
programming procedure:
Putting it all together
• try1.asm is a source file
o It is created by you, the programmer, using a text editor on your
PC
o Format: HC11 assembly language (ASCII text)
• try1.lst is a listing file
o It is created by AS11.exe, the assembler/linker that runs on your
PC
o (You'll probably run AS11.exe from DevHC11.exe)
o Its purpose of the file is to help you debug the program – Format:
ASCII text
Cont.
Putting it all together
• try1.s19 is an object file
o It is created by AS11.exe, the assembler/linker
o Its purpose is to specify what bytes should be downloaded to the
HC11, and to what locations in HC11 memory
o Format: Motorola S-record (ASCII text)
o Sometimes this is called a hex file
• The object file is converted to executable machine code
o The machine code is created by DevHC11.exe, the development
environment that runs on your PC
Cont..
Putting it all together
o Its purpose is to provide instructions and data to the HC11
Format: HC11 machine language (binary)
o Sometimes this is called the binary code
o The machine code is transferred to the HC11 through the
coordinated efforts of DevHC11.exe (running on your PC) and the
Buffalo monitor program (running on the HC11)
Programmer’s Model
• What features of the
processor are most
important to the
assembly-language
programmer?
o
o
o
o
Register set
Memory organization
Instruction set
Addressing modes
• Here is the register set
(again):
Cont.
Programmer’s Model
• Accumulators
o A: 8-bit general purpose accumulator
o B: 8-bit general purpose accumulator
o D: Double accumulator (concatenation of A and B for 16-bit
operations)
 Most operations can be done using either accumulator A or B
• Index registers
o X: 16-bit index register
o Y: 16-bit index register
 X and Y are used for indexed addressing
 X is preferred, usually, because addressing with Y is slower
and takes 1 extra byte of object code than with X
Cont.
Programmer’s Model
Operations on index registers:
o Simple operations (INC, DEC, and ADD from B) can be
performed
o More complex operations are done by exchanging the index
register and the D register, doing some computation, and then
exchanging the values again
o X or Y is often loaded with the base address of the I/O register
address space ($1000)
Cont.
Programmer’s Model
• SP: 16-bit stack pointer
 Stack may be anywhere in
the 64 Kbyte address space
 The stack grows downward
in memory (i.e., a push
decrements SP)
Cont.
Programmer’s Model
• PC: 16-bit Program Counter
• CCR: -bit Condition Code Register
o H, N, Z, V, C: rithmetic status bits
 N: Negative result
 Z: Zero result
 V: Overflow result
 C: Carry out from operation
 H: Carry from low nibble (4 bits) of accumulator
o S: Stop bit disabled
 =1 disables STOP instruction
 =1 after processor reset
Cont…
Programmer’s Model
o I: Interrupt mask
 =1 masks all maskable interrupts (not XIRQ)
 =1 after processor reset
o X: XIRQ interrupt mask
 =1 masks XIRQ
 =1 after processor reset (must be cleared just after reset)
Overview of the 68HC11
instruction set
• The instruction set specifies the kinds of data transfers
and transformations that can occur in the machine
• Instructions can be grouped into 5 broad categories
o Data transfers: instructions that move data to and between
registers
o Logical: instructions that perform logic operations on data --AND,
OR, etc.
o Arithmetic: addition, subtraction, increment, etc.
o Flow control: instructions that change the sequence of execution
of a program --conditional and unconditional branches, stack
operations, etc.
o Input / Output operations
Cont.
Overview of the 68HC11
instruction set
• An instruction generally consists of an opcode and
some operand(s)
• HC11 instructions are of different lengths (1 to 5 bytes)
• The instruction set is summarized in Table A.1 of the
text, Appendix A of the HC11 Reference Manual, and
M68HC11 E Series Programming Reference Guide
o Lists instruction mnemonic, brief description of the operation,
addressing modes available, machine code format of the
instruction, timing information, and effect on condition code
registers
Cont.
Overview of the 68HC11
instruction set
Cont.
Overview of the 68HC11
instruction set
• Opcode construction
o In general, an n-bit opcode is capable of specifying any one of 2n
instructions
o With 8-bit opcodes, 256 distinct instructions are possible
o 68HC11 has more than 300 actual instructions
 145 "basic" instructions plus “addressing variations”
 1 or 2 bytes of storage specify the opcode!
• 1-byte opcodes
o Most opcodes use just 1 byte
o Downward compatible with 6801
Cont.
Overview of the 68HC11
instruction set
• 2-byte opcodes
o Most 2-byte instructions deal with register Y, which was not
present in the 6801 processor
o It takes longer to fetch and execute 2-byte opcodes
o New instructions use a "pre-byte", which signals that another
opcode byte follows:
$18, $1A, or $CD
Ex. INX $08
INY $18 $08
Cont.
Overview of the 68HC11
instruction set
• Instruction format
o An instruction is made up of an opcode and a set of operands
 Opcode may be one or two bytes
 Instructions may use 0, 1, 2, or 3 operands
• Operands may be 1 or 2 bytes
o Instruction lengths range from 1 to 5 bytes
o Example (assume this is stored at address $E000):
LDAA #$FF
; load ACCA with the
; Value $FF
Machine code:
$E000 $86
$E001 $FF
Cont.
Overview of the 68HC11
instruction set
• Fetch/Execute
operation:
LDAA #$FF
First clock cycle
Cont.
Overview of the 68HC11
instruction set
• Fetch/Execute
operation:
LDAA #$FF
Second clock cycle
Addressing modes
• How does the instruction specify the location of data?
• The HC11 supports 5 different addressing modes
o Inherent Addressing
 Operands are specified implicitly in the opcode itself
 Operands are contained in the registers instead of memory
 Instruction length: or 2 bytes
 Examples:
• Register increment, clears
CLRA
• Register shift operations
ASLA
• Register additions
ABA
Cont.
Addressing modes
• How does the instruction specify the location of data?
• The HC11 supports 5 different addressing modes
o Inherent Addressing
 Operands are specified implicitly in the opcode itself
 Operands are contained in the registers instead of memory
 Instruction length: or 2 bytes
 Examples:
• Register increment, clears
CLRA
• Register shift operations
ASLA
• Register additions
ABA
Cont.
Addressing modes
• Immediate addressing
 One operand is included as part of the instruction word
 Other operand (if needed) comes from an implied register
 Instruction length: 2 -4 bytes
 Specified operand is a constant and cannot be altered at run
time
 Mode is denoted by a # before the operand value
 Example:
LDAA #$05
$86 $05
Cont.
Addressing mode
• Direct addressing
 The operand included in the instruction word is a memory
address, specifying where the data is located
 Second operand is an implied register
 Instruction length: 2 -4 bytes
 Since an address is specified, the data value itself can be
changed during program execution
• Address is a constant
 Address included in the instruction word is only 1 byte
• Direct addressing can only be used to reference locations
$0000 --$00FF --256 locations
Example:
LDAA 05
$96 $05
Cont.
Addressing mode
• Extended addressing
 Same as direct addressing, but with 2 bytes of address --the full
range of addresses can be specified ($0000 to $FFFF)
 Instruction length: 3 or 4 bytes
 Example:
LDAA $0005
$B6 $00 $05
 Why have both modes?
Cont.
Addressing mode
• Indexed addressing
 The effective address of the data in memory is
computed as the sum of the value contained in an
index register (X or Y) and an offset (contained in
the instruction word)
 The offset is a 1-byte unsigned quantity » Useful
for table and array access
 Eg.
DAA $56,X
$A6 56
Cont.
Addressing mode
• Relative addressing
 Similar to indexed addressing uses PC value
instead of the value in X or Y
 Instruction's offset value is added to the value in
the program counter to give the address of the next
instruction
 Used to specify branch addresses resulting from
jump commands
 Offset is an 8-bit 2's complement number --ranges
from -128 to +127 decimal
Basic tour of the
instruction set
• Load Instructions
o
o
o
o
Transfer data from memory to register
LDAA, LDAB, LDD, LDX, LDY, LDS
Different addressing modes supported
Assume the following memory contents:
0050 40 41 42 43 44 45 46 47 -48 49 4a 4b 4c 4d 4e 4f
2000 50 51 52 53 54 55 56 57 -58 59 5a 5b 5c 5d 5e 5f
LDAA
LDAB
LDAA
LDD
#$56
$56
$2000
$2002
;
;
;
;
ACCA$56 (immediate)
ACCB = $46 (direct)
ACCA = $50 (extended)
ACCD = $5253 (extended)
Cont.
Basic tour of the
instruction set
LDX #$2000
LDAA $C, X
LDY #$56
LDAB 0, Y
LDX $5, X
; CCA = $52, ACCB = $53
; IX = $2000 (immediate)
; ACCA = $5C (indexed)
; IY = $56 (immediate)
; ACCB = $46 (indexed)
; IX = ? (indexed)
• Store Instructions
o Transfer data from register to memory
o STAA, STAB, STD, STX, STY, STS
o Direct, extended, or indexed addressing No immediate
addressing
Cont.
Basic tour of the
instruction set
• Transfer Instructions
o Transfer data from register to register
TAB ; ACCA � ACCB
TBA ; ACCB � ACCA
TAP ; ACCA � CCR
TPA ; CCR � ACCA
TXS ; IX � SP
TSX ; SP � IX
TYS ; IY � SP
TSY ; SP � IY
XGDX ; IX � ACCD
XDGY ; IY � ACCD
Cont.
Basic tour of the
instruction set
• Increment Instructions
INC opr
INCA
INCB
INX
INY
INS
• Decrement Instructions
DEC opr
DECA
DECB
Cont.
Basic tour of the
instruction set
DEX
DEY
DES
• Clear Instructions
CLR opr
CLRA
Rotate Instructions
o Shifts all 8 bits plus the Carry bit
circularly one position.
ROL opr
ROLA
ROLB
Cont.
Basic tour of the
instruction set
ROR opr
RORA
RORB
o Note: 9 rotates puts
data back in original
position (not 8 as stated
in text)
• Rotate Instructions
Example
Assume C = 1, ACCA =
$3B
Cont.
Basic tour of the
instruction set
• Shift Instructions
o Logical
Shifts
LSL opr
LSLA
LSLB
LSLD
LSR opr
LSRA
LSRB
LSRD
Cont.
Basic tour of the
instruction set
• Shift Instructions
o Arithmatic
Shifts
ASL opr
ASLA
ASLB
ASLD
ASR opr
ASRA
ASRB
ASRD
 ASR preserves sign bit
 ASL/ASR can be used to
multiply/divide by 2
 What is the difference
between ASL and LSL?
Cont.
Basic tour of the
instruction set
• Logical Instructions
o Bit-wise AND
ANDA opr
ANDB opr
o Bit-wise OR
ORA opr
ORB opr
o Bit-wise Exclusive-OR
EORA opr
EORB opr
Cont.
Basic tour of the
instruction set
o 1’s Complement
COM opr
COMA
COMB
• Arithmetic Instructions
o Add instructions
ABA ; ACCA + ACCB � ACCA
ADDA opr ; ACCA + M � ACCA
ADDB opr ; ACCB + M � ACCB
ADDD opr ; ACCD + M � ACCD
ADCA opr ; ACCA+ M+ C � ACCA
Cont.
Basic tour of the
instruction set
ABX ; IX + ACCB � IX
ABY ; IY + ACCB � IY
o Subtract Instructions
SBA ; ACCA -ACCB � ACCA
SUBA opr ; ACCA -M � ACCA
SUBB opr ; ACCB -M � ACCB
SUBD opr ; ACCD -M � ACCD
SBCA opr ; ACCA -M -C � ACCA
SBCB opr ; ACCB -M -C � ACCB
Cont.
Basic tour of the
instruction set
• Arithmetic Instructions
o Unsigned arithmetic
 Numbers range from $00 to $FF (0 to 255)
 The carry bit (C) is set if result is outside range
o Signed arithmetic
 Numbers range from $80 to $7f (-128 to +127)
 The overflow bit (V) is set if result is outside range
 The negative bit (N) is set if result is negative (same as
MSB of result)
Cont.
Basic tour of the
instruction set
• Arithmetic instructions
o The same instructions are used for both signed and unsigned
arithmetic
 The CPU does not care whether the values are signed or
unsigned
• The arithmetic result is the same for both cases
• C, V, and N bits are set based on Boolean combinations of the
operands
 Programmer must keep track of whether values are signed
or unsigned
• Whether to use the C or V bit to check for overflow
Cont.
Basic tour of the
instruction set
• Arithmetic Instructions
o Example:
 Add $56 + $B0
• Unsigned: $56 + $B0 = $106 (86 + 176 = 262)
• Signed: $56 + $B0 = $06 (86 + (-80) = 6)
LDAA #$56
LDAB #$B0
ABA
;ACCA = $06, C = 1, V = 0
o Example:
 Add $56 to $60
• Unsigned: $56 + $60 = $B6 (86 + 96 = 182)
• Signed: $56 + $60 = $B6 (86 + 96 = -74!!!)
Cont.
Basic tour of the
instruction set
LDAA #$56
LDAB #$60
ABA
;ACCA = $B6, C = 0, V = 1
• Multiplication
o MUL instruction
 Multiplies 8-bit unsigned values in ACCA and ACCB,
then puts the 16-bit result in ACCD
• Note that this overwrites ACCA and ACCB!
o If you’re multiplying by a power of 2, you may want to use
ASL instructions instead
 Why?
Cont.
Basic tour of the
instruction set
• Binary Fractions
o Multiplying two 8-bit numbers gives 16-bit result. What if
you only want 8 bits?
o Different ways to interpret 8-bit numbers
 Unsigned integers 0 –255
 Signed integers -128 -+127
 Binary fractions 0 -1
• $00 = 0/256 = 0
• $80 = 128/256 = 0.5
• $FF = 255/256 = 0.9961
o You can use the ADCA instruction after a MUL to convert 16bit result to 8-bit binary fraction
 MUL sets C to 1 if bit 7 of ACCB is 1
Cont.
Basic tour of the
instruction set
• Binary Fractions
o Example:
 Multiply $20 x $35 (32 x 53 in decimal)
 Unsigned integers:
• $20 x $35 = 32 x 53 = 1696 = $6A0
LDAA #$20
LDAB #$35
MUL
; ACCD = $06A0, C = 1
; ACCA=$06, ACCB=$A0
 Binary fractions:
• (32 / 256) x (53 / 256) = 1696 / 65,536
Cont.
Basic tour of the
instruction set
= 6.625 / 256
= 7 / 256
LDAA #$20
LDAB #$35
MUL
; ACCD = $06A0, C = 1
ADCA #$00
; now ACCA = $07
• Integer Division
o IDIV
 Divides 16-bit unsigned integer in ACCD by 16-bit
unsigned integer in IX
 Puts quotient in IX, remainder in ACCD
o Example: 6 / 4
Cont.
Basic tour of the
instruction set
LDD #$6 ; ACCD = $6
LDX #$4 ; IX = $4
IDIV ; IX = $1, ACCD = $2
o Note: IDIV takes 41 cycles to execute
 You can use ASR instructions to divide by powers of 2
o Only gives you the quotient, not the remainder
o Divide by zero
 IX set to $FFFF, C set to 1
• Fractional Division
o FDIV
 Divides 16-bit binary fraction in ACCD by 16-bit fraction
in IX
• 16-bit values between 0 and 1
Cont.
Basic tour of the
instruction set
• 16-bit values between 0 and 1
 Puts quotient in IX, remainder in ACCD
 Often used to convert remainder from IDIV into a
fraction
 Example: 6/4
LDD #$6
LDX #$4
IDIV
; IX = $1, ACCD = $2
STX Somewhere ; store quotient
LDX #$4
; reload denominator
FDIV
; IX = $8000, ACCD = $0
; (Note $8000 = 0.5)
Cont.
Basic tour of the
instruction set
• Floating point numbers
o Used to increase the range of number representation past that
of integer formats
o Due to 8-bit CPU, IEEE floating point standard is not
supported on the HC11
o Motorola has a non-standard format
o Floating point routines can be downloaded from the Motorola
web site if you need these for projects (you will not need them
for the labs)
• CCR Operations
CLC
CLV
SEC
SEV
;
;
;
;
clear the
clear the
set the C
set the V
C bit
V bit
bit
bit
Cont.
Basic tour of the
instruction set
o Can use SEC and CLC to set up the C bit prior to rotate
instructions, for example
• “No Operation” Instruction
NOP
o Does nothing but increment the PC
o Why would you need this?
• Compare Instructions
o Compares two registers, or a register and a memory
location/immediate value
 Subtracts the two values
o Sets N, Z, V, C condition codes
o Does not change operands
Cont.
Basic tour of the
instruction set
CBA
; ACCA -ACCB
CMPA opr ; ACCA -M
CMPB opr ; ACCB –M
CPD opr ; ACCD -M
CPX opr ; IX -M
CPY opr ; IY -M
• Can be used to check if two values are equal, greater than/less
than, etc.
 Often used before conditional branch instruction
• Compare Instructions
• Example: Assume the 16-bit value $5100 is stored at memory
location $2000:
LDAA #$50
Cont.
Basic tour of the
instruction set
• Bit Set/Clear Instructions
o Used to set or clear multiple bits in a memory location
BSET opr mask
BCLR opr mask
o 1’s in mask specify which bits are set or cleared. Other bits
are unaffected.
o Example: Assume IX = $20, memory location $34 contains
the value $1A
BSET $14,X $31 ; now location $34 = $3B
BCLR $34 $1C
; now location $34 = $23
• Bit Test Instructions
Cont.
Basic tour of the
instruction set
• Bit Test Instructions
o Performs logical AND and modifies condition codes N and Z
(V is always cleared, C is unaffected)
o Does not modify the operands
BITA opr
BITB opr
o Example: Assume location $2000 contains the value $1A
LDAA #$3F
LDAB #$05
BITA #$03 ; N=0, Z=0 BITB $2000
; N=0, Z=1
Cont.
Basic tour of the
instruction set
• Flow control with
looping and branching
o Flow control is the
process of
 Making decisions
 Performing
operations in
sequence
 Repeating identical
operations
• Fig 2.21 Flow control
mechanisms
Cont.
Basic tour of the
instruction set
• Relative addressing
o Offset is added to the PC to cause the branch
o 8-bit offset
 Range is -128 to +127
o Destination address is actually relative to the address
following the branch instruction
 Why?
o Calculating destination addresses:
PCnew = PCold + T + rel
 PCold is address of branch instruction
 PCnew is destination address
 rel is the relative address
 T is 2 for most instructions
Cont.
Basic tour of the
instruction set
o T = 4 for BRCLR, BRSET with indexed addressing using IX
o T = 5 for BRCLR, BRSET with indexed addressing using IY
• Relative addressing
o The assembler usually handles the calculation of relative
addresses In your program, put a label on the destination
instruction, and then the branch instruction can reference the
label
o Example:
ORG $B600
Label: LDAA $100
INCA
STAA $100
BRA Label
;
;
;
;
get current val
increment it
store updated val
repeat
Cont.
Basic tour of the
instruction set
Object code:
B600 B6 01 00
B603 4C
B604 B7 01 00
B607 20 F7
• Conditional branching
o Here, the branch instruction checks if a (specified) condition is
true or false
 Uses condition codes in CCR
o Use branch instruction after setting the condition codes
 If it is true, branch to target address
 If false --no branch, continue execution at the next
instruction
o Uses only relative addressing to specify the target address
Cont.
Basic tour of the
instruction set
 Limited to 8-bit relative offset
 Textbook shows a way to combine a conditional branch
with a JMP to overcome this limit
• Conditional Branching
o Simple conditional branches
BCS rel ; branch if C=1
BCC rel ; branch if C=0
BMI rel ; branch if N=1
BPL rel ; branch if N=0
BVS rel ; branch if V=1
BVC rel ; branch if V=0
BEQ rel ; branch if Z=1
BNE rel ; branch if Z=0
Cont.
Basic tour of the
instruction set
• Conditional Branching
o Signed branches
 Use when working with signed values
 Branches are based on N, Z, and V bits only
BGT rel ; greater than
BLT rel ; less than
BGE rel ; greater than or equal
BLE rel ; less than or equal
BEQ rel ; equal
BNE rel ; not equal
• Conditional Branching
o Unsigned branches
 Use when working with unsigned values
 Branches based on C and Z condition codes
Cont.
Basic tour of the
instruction set
 Note that Motorola uses the terms “greater than” and “less
than” to refer to signed values, and “higher” and “lower” to
refer to unsigned values
BHI rel ; higher
BHS rel ; higher or same (same as BCC)
BLO rel ; lower (same as BCS)
BLS rel ; lower or same
BEQ rel ; equal
BNE rel ; not equal
 Conditional branching can also be based on bit values in a
particular memory value
o Format:
BRCLR opr mask rel
BRSET opr mask rel
Cont.
Basic tour of the
instruction set
o opr is the memory address (direct or indexed addressing is
used)
o mask identifies the bits to check (set to 1, others are 0) in an
AND format
o rel is the relative branch address
o Example:
BRSET $10 $05 new
Branch to address "new" if bits 0 and 2 of the value stored at
location $10 are both 1
• Fetch/Execute Operation
o BNE LOOP
o First clock cycle
Cont.
Basic tour of the
instruction set
Cont.
Basic tour of the
instruction set
• Fetch/Execute
Operation
o BNE OOP
o Second clock cycle
Cont.
Basic tour of the
instruction set
• Fetch/Execute
Operation
o BNE OOP
o Third clock cycle
o New PC = $E012 +
$FFF6 = $E008
Cont.
Basic tour of the
instruction set
o How do we stop a program once its execution is finished?
o What happens at the "end" of our program?
•
STOP
o This instruction stops the execution by disabling all system
clocks and putting the system in a minimum-power standby
mode
o Only ways to recover from a STOP are:
 Perform a system RESET*
 Generate a valid system interrupt
o Avoid using this –catastrophic
• BRA $ or BRA *
o Used in our text's examples
Cont.
Basic tour of the
instruction set
o Infinite loop branching back on itself
o $ (or *) denotes "current" address (location of BRA instruction)
o Program never really terminates, so use with caution
•
SWI
o This instruction causes a software generated interrupt to occur
o Upon occurrence, can jump to a location in memory and
execute what's found there
o Some systems use this approach to return to a monitor routine
after program execution is finished