Ch.2 Intro. To Assembly
Language Programming
From Introduction to Embedded
Systems: Interfacing to the
Freescale 9s12 by Valvano, published
by CENGAGE
2.1 Binary and Hexadecimal
Numbers
• Decimal numbers
– {0,1,2,…,7,8,9}
– A decimal number is a combination of digits multiplied
by powers of 10
– 1984 = (1)x103 + (9)x102 + 8 x101 + (4)x100
• Binary numbers
– {0,1}
– A binary number is a combination of binary digits
multiplied by powers of 2.
– 011010102 = %01101010
– 0x27+1x26+1x25+0x24+1x23+0x22+2x21+2x20
– = 0 +64 +32+0+8+2+0= 106
2.1 Binary and Hexadecimal
Numbers (cont.)
• Hexadecimal numbers
– {0,1,…9,A,B,C,D,E,F}, where A=10, …, F=15
– A hexadecimal number is a combination of the digits
{0-F} multiplied by powers of 16
– 12AD16 = $12AD = 1x163 +2x162 +(10)x161 +(13)x160
= 4096 + 512 + 160 +13=4781
•
•
•
•
Fractions—use negative powers.
Words=16 bits on the 9s12.
Bytes = 8 bits on the 9s12.
Nibble = 4 bits.
Checkpoints
• Checkpoint 2.1 What is the numerical value of
%11111111?
• Checkpoint 2.2 What is the numerical value of $FF?
• Checkpoint 2.3 Convert the binary number %01000101
to hexadecimal.
• Checkpoint 2.4 Convert the binary number
%110010101011 to hexadecimal.
• Checkpoint 2.5 Convert the hex number $40 to binary
• Checkpoint 2.6 Convert the hex number $63F to binary.
• Checkpoint 2.7 How many binary bits does it take to
represent $123456?
2.2 Addresses, Registers, and
Accessing Memory
• Figure 2.3—memory model of simplified
9S12 computer (small boxes—8 bits)
• Address—specifies the location from
where to read data to where to write data.
• Addresses—simple linear sequences—
from $0000 to $FFFF.
– $0240 points to an I/O port (Port T)
– $3800 points to a location in RAM
– $F004 points to a location in EEPROM
Registers
– Registers—high speed storage devices
located in the processor.
– Registers do not have addresses, but instead
have names and numbers.
• Register A (8 bits)
– Program Counter (PC)—16 bits—contains the
address of the instruction that is executing.
Read/Write Operations
• For this text (see page 30)
– =[U] specifies an 8-bit read from address U.
– ={U} specifies a 16-bit read from addresses U, U+1
(most significant byte first).
– [U]= specifies an 8-bit write to address U.
– {U}= specifies a 16-bit write to addresses U, U+1
(most significant byte first).
Checkpoint
• Checkpoint 2.8: What does [$0240] = RegA
mean literally? What is the overall action?
Registers of the 9S12
• Fig. 22.4 shows the six registers (page 30 of
text).
– Accumulator—typically used to hold and manipulate
numbers—RegA and RegB (8 bits each or 16 bits
when combined –RegD)
– Index Registers —RegX and RegY –16 bits each—
addresses and pointers.
– Program Counter (PC)—points to the current
instruction and is 16 bits.
– Condition Code Register (CC or CCR)—8 separate
bits—S, X, H, I, N, Z, V, C—Z is set of the result is
zero after an operation.
CLI
• The I bit can be cleared using the CLI
instruction during the debugging of a
program.
• More in Chapter 9.
Checkpoint
• Checkpoint 2.9 (page 31): Think about
how you could use the “subtract” and the
“branch on zero” instructions to test if two
numbers are equal?
Stack Pointer Register
• The stack is temporary storage in RAM.
• SP is a 16-bit register that points to the top
of the stack.
• Figure 2.5 illustrates the process of
“pushing” on the stack (the SP is
decremented and the data is written.)
• To “pop” data, the data is read from the
location specified by SP, then SP is
incremented.
Memory Maps
• Table 2.2—9S12C32
• Table 2.3 9S12DPS12
• Table 2.3 9S12E128
2.3 Assembly Syntax
• 2.3.1 Assembly Language Instructions
– Load AccumulatorA : ldaa
• Reads an 8-bit byte and places it into register A.
– Assembly code fields
• Label opcode
• Here ldaa
operand
$3800
comment
; RegA= $3800
Assembly Code and Machine Code
• Assembly language instructions, assembly code,
assembly source code
• Machine instructions, machine code, and object
code (can be loaded into EEPROM and
executed.)
• Example:
– Assembly code: ldaa $3800
– Machine code: $B6, $38, $00
• An assembler converts the assembly language
program into machine code.
2.3.2 Pseudo Operation Codes
• Pseudo-op, pseudo operation code, and
assembly directives are equivalent.
• These are used by the assembler during
the assembly process and are not
executed as assembly code.
• Example:
– org $4000 means that the machine code will
be loaded into EEPROM memory starting at
location $4000.
More Pseudo Codes
• equate is used to define a symbol.
• Reserve multiple bytes (rmb) is used to
define uninitialized variables.
• Form double byte (fdb) is used to form a
16 bit constant. The following two lines
indicate where to start execution:
•
org $FFFE
•
fdb main
Checkpoint
• Consider the following:
–
–
–
org $8080
Ptr1
rmb 2
Data1 rmb 1
• Ptr1 is found at $8080 and $8081.
• Checkpoint 2.10 Where in memory will
the variable Data1 be located?
• Hint: it comes after Ptr1; it is not in $8080.
2.4 Simplified 9S12 Machine
Language Execution
• A simplified cycle-by-cycle analysis is
done.
• TExaS can simulate both the hardware
devices and software action at the same
time.
• Table 2.5 (page 34) illustrates the
difference between a real 9S12 and the
TExaS bus cycle simulation.
Components of the 9S12
• See Figure 2.6 (page 34).
– Control Unit (CU) —controls the sequence of operations.
– Instruction Register (IR) —contains the op code of the current
instruction.
– Arithmetic Logic Unit (ALU) —performs addition, subtraction,
multiplication, division, and, or, and shift.
– Bus Interface Unit (BIU) —reads and writes data to the bus.
– Effective Address Register (EAR) –contains data address for
the current instruction –TExaS allows the EAR to be observed.
– Bus—8 bits and 16 bits (R/W is the control).
– Memory
– I/O Ports
Read and Write Cycles
• Read Cycles
– Instruction fetch —PC points to data to be placed in
the IR.
– Operand fetch — effective address is calculated.
– Data fetch –the address is in the EAR, and data is
placed in a register or sent to the ALU,
– Stack pull -- data is “popped” from the stack.
• Write Cycles
– Data write (from a register or ALU to memory
location stored in the EAR)
– Stack push
Phases of Execution (page 35 of
text)
•
•
•
•
•
•
Phase 1: Opcode and Operand fetch
Phase 2: Decode instruction
Phase 3: Evaluate address
Phase 4: Data read
Phase 5: Free cycles
Phase 6: Data write
Simple Address Modes
• Inherent Addressing
• Immediate Addressing
• Direct (Page) Addressing—other
computers call this zero page addressing.
• Extended Addressing—other computers
call this direct addressing.
• Indexed Addressing
• PC Relative Addressing
Addressing Modes
• 2.5.1 Inherent Addressing Mode
– There is no operand field.
– Example: clra (machine code $87)
– Figure 2.7 The data for the instruction is implied—
RegA is set to zero.
• 2.5.2 Immediate Addressing Mode
– The data is included in the machine code.
– Example ldaa #36 (machine code $86 $24)
– Figure 2.8—phase 1: fetch opcode; phase 2 fetch
operand.
Addressing Modes
• 2.5.3 Direct (Page) Addressing
– Uses an 8-bit address to access location from 0 to
$00FF (some addresses are I/O ports)
– Example: ldaa $32 (machine code: $96 $32)
– Figure 2.9 –opcode is read; operand is read; value of
Port K ($32) is $57; $57 is placed in register A.
• 2.5.4 Extended Addressing
– Uses a 16-bit address; allows access to all memory
and I/O.
– Example: ldaa $3800 (machine code: $B6 $38 $00
– Figure 2.10 --opcode is read; operand is read; rest of
operand is read; address is calculated and data is
moved into register A.
Addressing Modes
• 2.5.5 Indexed Addressing
– Uses a 16-bit pointer in a register to access memory and I/O
devices.
– RegX and RegY can be used as the pointers.
– Example: ldaa 0,x (machine code: $A6 $00)
– Figure 2.11—opcode is fetched; operand is fetched; fetch using
the EAR, the data in $3900.
• 2.5.6 PC Relative Addressing
–
–
–
–
Used for the branch and branch to subroutine instructions.
Example: bra main (machine code $20 $F4)
Fig. 2.11—opcode is fetched; operand is fetched.
The operand field for PC relative addressing is an 8-bit value
called rr.
– rr = (destination address) – (location of instruction) – (size of the
instruction).
Checkpoints
• Checkpoint 2.12: What is the difference
between ldaa #36 and ldaa #$24?
• Checkpoint 2.13: What is the difference
between ldaa #32 and ldaa $32?
• Checkpoint 2.14: Give the machine code
for the assembly code that branches to
itself, causing an infinite loop, loop bra
loop
2.6 The Assembly Language
Development Process
• An editor is used to create the source code.
• An assembler is used to translate the source
code to machine instructions.
• The assembler also produces a listing file,
which shows the addresses and object code that
corresponds to each lin of the assembly code.
• A loader is used to place the object code into
memory, when a real microcontroller is used.
2.7 Memory Transfer Operations
(page 41 of text)
• Symbols used
– w—signed 8-bit (-128 to + 127) or unsigned 8bit (0-255)
– n -- signed 8-bit
– u – unsigned 8-bit
– W -- signed 16 bit (-32787 to + 32767) or
unsigned 16- bit (0 to 65535)
– N -- signed 16-bit
– U -- unsigned 16-bit
Instructions for Memory
Transfer(page 42)
• Note that copies are made of the memory
contents.
• Instructions:
– Load (memory contents are copied into a
register.)
– Move (copies of memory values are moved
into other memory locations.)
– Store (register contents are copied and
moved into memory.)
Checkpoints
• Checkpoint 2.15: What is the difference
between ldx #$0801 and ldx $0801?
• Checkpoint 2.16: What is the difference
between direct mode instruction ldx<$12 and
the extended mode instruction ldx>$0012?
• Checkpoint 2.17: Write assembly code that
copies the 8-bit data from memory location
$0810 to memory location $0820.
• Checkpoint 2.18: Write assembly code that
writes the binary %11000111 to Port T.
2.8 Subroutines
• Subroutines are subprograms that may or may
not return a value (text—page 43).
• Program 2.1 (page 43) Subroutine Set
– Command bsr is used (but jsr could also be used.)
– Relative addressing is used with bsr and extended
addressing is used with jsr.
– At run time, the return address will be pushed on the
stack, and then will be pulled when the subroutine is
finished (instruction rts is used.)
– Two global variables: Flag and Data
– Fig. 2.13 illustrates the stack during execution.
2.9 Input/Output
• 2.9.1 Direction Registers
– PTT—8-bit Port T—address $0240.
– Each of the eight pins can be an input (logic
levels.)
– Fig. 2.15—illustrates the direction register
DDRT.
• If DDRT = $FF all pins are outputs.
• If DDRT = $00 all pins are inputs.
• If DDRT = $0F PT0-PT3 are outpus; others are
inputs.
Checkpoint
• Checkpoint 2.19: What happens if we
were to set DDRT ro $F0?
Example 2.1 (page 46 of text)
•
Make PTT pins 7-4 input and pins 3-0
output, then make PT3-PT0 output high.
1. Set the direction register:
– ldaa #$0F
– staa DDRT
2. Set the outputs high
– ldaa #$0F
– staa PTT
2.9.2 Switch Interface
• SPST—single poll single throw
– Figure 2.16 (page 47).
• Mechanical switches is subject to
bouncing—this can be reduced by reading
the value and then waiting 10ms before
reading again.
2.9.3 LED Interface
• LEDs emit light when electric current passes
through them.
• The current must pass from the anode to the
cathode.
• The cathode is the short lead.
• Figure 2.17 (page 47).
–
–
–
–
–
Suppose the desired brightness requires 1.9V.
R = (5 –Vd-Vol)/Id = (5-1.9-.5)/.01 = 260 ohms.
Vd is the desired operating voltage.
Vol is the ouput low voltage of the LED driver.
Id is the desired LED current.
Checkpoint
• Checkpoint 2.20: What resistor value in
Figure 2.17 is needed if the desired LED
operating point is 1.7 V and 5 mA?
LEDs (cont.)
• The driver is not needed when the LED
current is much less than 10 mA.
• Figure 2.18 (page 48) illustrates this
situation for LED interfacing.
– Positive logic interface: R = (Voh – Vd)/Id.
– Negative logic interface: R = (5-Vd-Vol)/Id.
Checkpoint
• Checkpoint 2.21: What resistor value in
Figure 2.18 is needed if the desired LED
operating point is 1.7V and 2 mA?
Example 2.2 (page 48-49)
• Build a system with three LEDs that flash
a rotating sequence 100,010,001 over and
over.
– Use low current LEDs (cheaper and easier to
interface).
– Use PT0-PT2 as the three output pins.
– As shown in Figure 2.18, build three positive
logic LED circuits as shown in Figure 2.19.
– Software is shown on pages 49 and 50.