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ee 308 spring 2003 68HCS12 addr ess space 68HCS12 has 16 address lines 68HCS12 can address distinct locations f or 68HCS12, each location holds one byte (eight bits) 68HCS12 can address bytes

kb ( 68HCS12 can address 64 kb 1
ee 308 spring 2003 68HCS12 addr ess space lo west address: highest address:

0x0000 0xffff 65535 0 2 ? 1 16 2
ee 308 spring 2003 memor y types ram: random access memory (can read and write) r om: read only memory (programmed at f actory) pr om: programmable read only memory (program once at site) epr om: erasable programmable read only memory (program at site, can erase using uv light and reprogram) eepr om: electrically erasable programmable read only memory (program and erase using v oltage rather than uv light) 68HCS12 has: 12 k ram 4 kb eepr om can erase and reprogram an y byte using normal 5v po wer supply 256 kb flash eepr om can erase using e xternal 12v po wer supply 3
ee 308 spring 2003 0x0000 registers 0x03ff 1 k bytes 68HCS12 address space 0x1000 0x0400 0x0fff 0x3fff d?bug 12 ram 0x4000 0xffff d?bug 12 flash eeprom user ram 0x3bff 0x3c00 1 k 11 k 3 k bytes eeprom 4
ee 308 spring 2003 68HCS12 alu arithmetic logic unit (alu) is where instructions are e x ecuted. examples of instructions are arithmetic (add, subtract), logical (bitwise and, bitwise or), and comparison. 68HCS12 has tw o 8-bit re gisters for e x ecuting instructions. these re gisters are called a and b . f or e xample, the hcs12 can add the 8-bit number stored in b to the eight-bit number stored in a using the instruction ab a (add b to a): combinational block a b clock (add b to a) control (0x1806 = aba) when the control unit sees the sixteen-bit number 0x1806 , it tells the alu to add b to a , and store the result into a . 5
ee 308 spring 2003 68HCS12 pr ogramming model a programming model details the re gisters in the alu and con- trol unit which a programmer needs to kno w about to program a microprocessor . re gisters a and b are part of the programming model. some instructions treat a and b as a sixteen-bit re gister called d for such things as adding tw o sixteen-bit numbers. note that d is the same as a and b . 0 b d 0 0 15 a 7 7 the hcs12 can w ork with 8-bit numbers (bytes) and 16-bit numbers (w ords). the size of w ord the hcs12 uses depends on the instruction. f or e xample, the instruction ld aa (load accumulator a) puts a byte into a , and ldd (load double accumulator) puts a w ord into d . 6
ee 308 spring 2003 68HCS12 pr ogramming model the 68HCS12 has a sixteen-bit re gister which tells the control unit which instruction to e x ecute. this is called the program counter ( pc ). the number in pc is the address of the ne xt in- struction the hc12 will e x ecute. the 68HCS12 has an eight-bit re gister which tells the hcs12 about the state of the alu. this re gister is called the condition code re gister (ccr). f or e xample, one bit (c) tells the hcs12 whether the last instruction e x ecuted generated a carry . another bit (z) tells the hcs12 whether the result of the last instruction w as zero. the n bit tells whether the last instruction e x ecuted generated a ne g ati v e result. there are three other 16-bit re gisters C x, y , sp C which we will discuss later . 0 15 0 15 0 15 0 15 0 b d x y sp pc ccr 0 0 15 a 7 7 n i h x s z v c 7
ee 308 spring 2003 some hcs12 instructions needed f or lab 1 ldaa address put the byte contained in memory at address into a staa address put the byte contained in a into memory at address clra clear a (0 -> a ) inca add 1 to a (( a ) + 1 -> a ) aba add b to a , store the result in a asra shift a right by one bit (k eep the msb the same) this di vides a signed byte by 2 lsra shift a right by one bit (put 0 into msb) this di vides an unsigned byte by 2 nega ne g ate a (-( a ) -> a ) tab t ransfer a to b (( a ) -> b ) swi softw are interrupt (used to end all our hcs12 programs) 8
ee 308 spring 2003 a simple hcs12 pr ogram all programs and data must be placed in memory between ad- dress 0x1000 and 0x3bff . f or our short programs we will put the ?rst instruction at 0x1000 , and the ?rst data byte at 0x1100 consider the follo wing program: ldaa $1113 ; put contents of memory at 0x1113 into a inca ; add one to a staa $1114 ; store the result into memory at 0x1114 swi ; end program if the ?rst instruction is at address 0x1000 , the follo wing bytes in memory will tell the hcs12 to e x ecute the abo v e program: address value instruction 0x1000 b6 ldaa $1113 0x1001 11 0x1002 13 0x1003 42 inca 0x1004 7a staa $1114 0x1005 11 0x1006 14 0x1007 3f swi if the contents of address 0x1113 were 0xa2 , the program w ould put an 0xa3 into address 0x1114 . 9
ee 308 spring 2003 a simple assembly language pr ogram. it is dif ?cult for humans to remember the numbers (op codes) for computer instructions. it is also hard for us to k eep track of the addresses of numerous data v alues. instead we use w ords called mnemonics to represent instructions, and labels to repre- sent addresses, and let a computer program called an assembler to con v ert our program to binary numbers (machine code). here is an assembly language program to implement the pre vi- ous program: prog equ $1000 ; start program at 0x1000 data equ $1113 ; data value at 0x1113 result equ $1114 ; result at 0x1114 org prog code: section .text ldaa data inca staa result swi w e w ould put this code into a ?le and gi v e it a name, such as test.s note that equ , org , and section are not instructions for the hcs12 b ut are directi v es to the assembler which mak e it possi- ble for us to write assembly language programs. the y are called assembler directi v es or pseudo-ops. f or e xample the pseudo-op org tells the assembler that the starting address (origin) of our program should be 0x1000 . 10
ee 308 spring 2003 assembling an assembly language pr ogram a computer program called an assembler can con v ert an assem- bly language program into machine code. the assembler we use in class is from a compan y called cosmic. t o assemble the abo v e program using the cosmic assembler , we must ?rst create a ?le called test.lkf which tells the assem- bler where to put things in memory . our test.lkf ?le w ould look lik e this: # link file for test program +seg .text -b 0x1000 -n .text # program start address +seg .data -b 0x1100 -n .data # data start address test.o # application program t o assemble the program, use the follo wing commands: ca6812 -a -l -xx -pl test.s clnk -o test.h12 -m test.map test.lkf chex -o test.s19 test.h12 this will produce a ?le called test.s19 which we can load into the 68HCS12. 11

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