Assembly Language

1
Assembly Language

• We can use our current knowledge of the HC11 to
  hand assemble a program
• Select operation
• Select instruction mode
• Get opcode from table
• Determine operands
• etc.
• This is a laborious process
• Some simple tools will make programming at this
  low level much more effective
• Some notion of variables (labels)
• Automatic lookup of opcode from table
• Rather use mnemonic of operation

2
Example

• We are using memory location 0x25 to store a
  value that we want to load into accumulator A
• Hand assemble
• Look up opcode for direct (or extended) mode load
  the accumulator 0x96
• Determine operand 0x25 in this case
• 0x96 0x25
• Easier to
• LDAA 0x25
• Even better to
• define myvar 0x25
• LDAA myvar

3
Assembly With Mnemonics

• For the most part, very straightforward
• Operations with only one addressing mode can be
  referred to with mnemonic without confusion.
• Inherent, Relative
• Operations with multiple addressing modes need
  help
• LDAA 0x25 ? immediate mode loads 0x25 into A
• LDAA 0x25 ? direct mode load whatever is in
  address 0x0025 into A
• LDAA 0x1425 ? extended mode load whatever is in
  address 0x1425
• LDAA 0x03,x ? indexed with X
• LDAA 0x03,y ? indexed with X
• Some assemblers get confused
• LDAA 0x0025 ? direct or extended?
• Should automatically optimize. Some do, some do
  not.
• forces direct mode GNU.
• LDAA 0x25
• Mostly an issue when labels are used (up next).

4
Labels a.k.a Symbols

• Act as variables and/or placeholders in assembly
  source
• Resolved during assembly
• Properties of Regular Labels GNU
• Not limited in length
• Must be unique in source file
• Must be unique in project if made global
• Must begin with A-Z a-z . _
• Subsequent characters 0-9 A-Z a-z _
• Colon terminates ()

5
Local Labels GNU

• Primarily used for branches
• 1 to maxPositiveInteger
• Scope
• Back to same label or beginning of file
• Forward to same label or end of file
• Must specify whether you want the
• Next (f forward) matching label
• Previous (b backward) one
• If no fb, number is interpreted as a relative
  addressing mode offset.

6
Sections

• A type of label that identifies a section or
  range of memory
• .section name
• Standard sections GNU
• .bss uninitialized mutable data (block started
  by symbol
• .data initialized mutable data
• .rodata initialized read-only data
• .text code

7
Data Storage

• Directives can follow label declarations to
  create data elements
• label directive data , data,
• Reserve but do not initialize
• Byte .space N, .rmb N
• Word .ds N
• Reserve and initialize
• Byte .byte 0x01, , .fcb 0x05,
• Word .word 0x1234,
• String
• .ascii string stores characters
• .asciz string stores characters plus null
  terminator
• Data in .bss will not be initialized, that is, it
  will not be included in download image, even if
  you use .byte, .word, etc.

8
A Couple More Directives

• .global label create a global label
• For linking multiple modules
• Linker will be looking for a global label
  _start to know where to start execution GNU
• .include name include another source file
• sets off a comment to the end of the line
• Note, works with labels as well
• LDAA mydata ? loads whatever value mydata is
• LDAA mydata ? loads whatever value is stored at
  address mydata

9

• MAXLEN 128
• .section .bss bss uninitialized data --
  maps data section
• (starting at 0xF000) per fox11w.x
• length .short 1 reserve a 16-bit word for
  the length (uninitialized)
• .section .rodata
• string .asciz "Testing... one... two...
  three..."
• .section .text
• .global _start exported entry point expected
  by GNU as
• _start
• Set up string pointer and counter
• ldx string pointer to string
• ldy 0 count of non-null
  characters

10
Listing File

• lab1.elf file format elf32-m68hc11
• Contents of section .text
• 8400 ce841c18 ce00006d 00270b18 08188c00
  .......m.'......
• 8410 80270308 20f118ff f00020fe .'..
  ..... .
• Contents of section .rodata
• 841c 54657374 696e672e 2e2e206f 6e652e2e
  Testing... one..
• 842c 2e207477 6f2e2e2e 20746872 65652e2e .
  two... three..
• 843c 2e00
• ..
• Disassembly of section .text
• 00008400 lt_startgt
• 8400 ce 84 1c ldx 841c lt_etextgt
• 8403 18 ce 00 00 ldy 0 lt__data_section_size
  gt
• 8407 6d 00 tst 0,x
• 8409 27 0b beq 8416 lt_start0x16gt
• 840b 18 08 iny
• 840d 18 8c 00 80 cpy 80 ltMAXLENgt

11
Questions