Writing a Simple Program in an Assembly Language

Chapter Writing a Simple Program in an Assembly Language This chapter gives an overview of a program developed in an assembly language used by the H8/300H Only basic instructions are introduced here to help you understand how a program proceeds and how the contents of the registers and memory change as it progresses To understand these subjects, you need a knowledge of binary numbers described in Chapter Learn how a program is configured and proceeds before going on to the next chapter which explains instructions in detail The assembly language is the most basic programming language and corresponds to machine instructions one-to-one, making it the most suitable language for understanding microcomputer operation Although C-language is also becoming popular in the microcomputer field, studying programs written in the assembly language will be very helpful for developing a program with C-language afterward The CPU can execute machine instructions only No matter in which language a program is written, it must be converted into machine instructions in the end Since machine instructions are collections of 0s and 1s, it is difficult to develop a program directly with machine language For this reason, assembly language is used since it enables machine language to be expressed in easily understandable alphabets For example, a machine instruction to add the R0L and R1L as an arithmetic register is expressed as follows in 16 bits: 0000 1000 1000 1001 In assembly language, it is expressed as follows: ADD.B R0L,R1L A program written in assembly language is referred to as a source program 4.1 CPU Internal Registers Before developing a program with assembly language, you need to know what kinds of registers and functions the CPU has Figure 4.1 shows the CPU internal registers of the H8/300H http://resource.renesas.com Page 29 Figure 4.1: CPU Internal Registers The internal registers are classified into general-purpose and control registers The general-purpose registers are used to calculate data and store addresses The control register is further classified into the PC (program counter) to control program progress and the CCR (condition code register) to test conditions How to use general-purpose registers The CPU has general-purpose registers, each capable of storing 32digit binary numbers.In addition to 32-bit data, they can also store 16- or 8-bit data When 32-bit data is stored, they are described as follows in an instruction, using registers in all: ER0, ER1, ER2, ER3, ER4, ER5, ER6, ER7 When 16-bit data is stored, they are described as follows in an instruction, using registers as 16 units in all: E0, E1, E2, E3, E4, E5, E6, E7, R0, R1, R2, R3, R4, R5, R6, R7 When 8-bit data is stored, they are described as follows in an instruction, using registers as 16 units in all: R0H, R0L, R1H, R1L, R2H, R2L, R3H, R3L, R4H, R4L, R5H, R5L, R6H, R6L, R7H, R7L http://resource.renesas.com Page 30 This is illustrated in Figure 4.2 Figure 4.2: General-purpose Register Example of calculation by general-purpose registers In this example, an add instruction is used to show how general-purpose registers are actually used ADD.B R0L,R1H is an instruction for 8-bit addition ADD represents "ADDition" and B represents "Byte" (8 bits) The contents of the R1H and R0L are added and the results are stored in the R1H This will not influence the E1 or R1L Only 8-bit results are obtained Any 8-bit register is available for this calculation For example, you can specify the same register like "ADD.B R1L,R1L" In this case, the R1L is doubled ADD.W R0,E1 is an instruction for 16-bit addition ADD represents "ADDition" and W represents "Word" (16 bits) The contents of the E1 and R0 are added and the results are stored in the E1 http://resource.renesas.com Page 31 This will not influence the R1.Only 16-bit results are obtained ADD.L ER0,ER1 is an instruction for 32-bit addition.ADD represents "ADDition" and L represents "Long word" (32 bits) The contents of the ER1 and ER0 are added and the results are stored in the ER1 The 32-bit results are stored in the ER1 SP (stack pointer) A special function has been added to the ER7 as a stack pointer The ER7 is usually not used for calculation but as a stack pointer The stack pointer function is described in detail in "Subroutines" and "Interrupt Operations" PC (program counter) n the program counter, the "address of the instruction to be executed next" is always stored and the data is automatically updated every time the CPU reads instructions Since the addresses are 24 bits, the PC also has 24-bit configuration Programmers need not pay special attention to how the PC is configured Every time an instruction is read, the address of the next instruction is automatically stored In the case of the H8/300H, an instruction is always read from an evennumbered address first This means that an even-numbered address is always stored in the PC (see "Data in the memory") CCR (condition code register) This is used to control interrupts or test conditions Although it is an 8bit register, every bit has a different meaning Interrupt control is described in detail in "Exception Handling" This section describes the part used for conditional test Every time an instruction is executed, the N, Z, V and C bits change to reflect the results Conditions are tested based on their changes An instruction to be tested exists separately The N, Z, V and C bits are called "flags" N (Negative) flag: When the execution results are regarded to be a signed binary number, set to if it is negative, or if positive Z (Zero) flag: Set to if the execution results are zero, otherwise, V (oVerflow) flag: When the execution results are regarded to be a signed binary number, set to if it overflows, otherwise, C (Carry) flag: Set to if execution results in a carry or borrow, otherwise, http://resource.renesas.com Page 32 Conditional test in a program is performed by these four flags Any condition can be tested using them Conditional test using the CCR As for two numeric values, X and Y, let's consider how to test their collating sequence.To test the collating sequence, subtraction is used By subtracting Y from X, the sequence can be tested based on how N, Z, V and C in the CCR change Let's assume that C becomes after subtracting Y from X This means that a borrow occurred after subtraction A borrow occurs when X is less than Y If a borrow does not occur, you can judge that X is equal to or greater than Y If Z is 1, X is equal to Y since the subtraction results are zero.If Z is 0, you can judge that X is not equal to Y As described above, the collating sequence can be tested based on C and Z obtained after subtraction In this case, however, X and Y are assumed to be unsigned binary numbers If they are signed binary numbers, the N or V flag, instead of C, is used for conditional test Data in the memory The following describes how to store 8-, 16- and 32-bit data into the memory Not only the H8/300H but all 16-bit microcomputers use bits of the memory per address So, one 8-bit data block exactly occupies one address One 16-bit data block occupies two addresses The upper bits are stored in a smaller address and the lower bits in a larger one The smaller one must be an even-numbered address Although each data block is stored separately in two addresses, the smaller one is regarded to be the address storing the data For example, "16-bit data in the H'1000 address" means that the upper bits are stored in the H'1000 address and the lower in the H'1001 address http://resource.renesas.com Page 33 In an instruction to read or write 16-bit data, you should specify an even-numbered address (smaller address) If you attempt to read or write 16-bit data by specifying an odd-numbered address, reading/writing will fail For the reason why this restriction applies, refer to "Connecting CPU to Memory (16bit Data Bus)" In the case of the H8/300H, an instruction is always read in 16-bit units This means that an instruction must be stored in an even-numbered address H8/300H machine instructions are composed in 16-bit integral multiples If the first instruction falls in an even-numbered address, the subsequent instructions also fall in even-numbered addresses One 32-bit data block occupies four addresses of the memory Since the H8/300H cannot read or write 32-bit data at a time, data are divided into 16-bit units for reading/writing In this case, the first data must also fall in an evennumbered address Likewise, the most significant bits are stored in the smallest address and the least significant bits in the largest one [Explanation with motion pictures and sound] http://resource.renesas.com Page 34 ( T ) There are eight 32-bit general-purpose registers in all There are eight general-purpose registers, from ER0 to ER7 ( T ) The ER7 is a stack pointer Among the general-purpose registers, only the ER7 has a special stack pointer function ( F ) The CCR is a 16-bit register The CCR (Condition Code Register) is a control registers with 8-bit configuration ( F ) The PC stores the instruction currently being executed The PC (Program Counter) does not store instructions but the "address" of the instruction to be executed next ( F ) Although the ER0 can perform addition, the ER6 cannot All general purpose registers from ER0 to ER7 can handle the same instructions.(The ER7, however, has a special stack pointer function) ( T ) The least significant bits of the ER0 is the R0L The upper 16 bits of the ER0 are the E0 and the lower 16 bits are the R0.And the upper bits of the R0 are the R0H and the lower bits are the R0L ( F ) The upper 16 bits of the ER0 is the R0 The upper 16 bits of the ER0 are the E0 and the lower 16 bits are the R0 ( F ) The Z flag in the CCR is set to zero when calculation results in zero Since this flag is named "Zero", it is set to when calculation results in zero ( T ) The N flag in the CCR is set to zero when the calculation results are positive Since this flag is named "Negative", it is set to when the calculation results are negative Zero when positive 10 ( F )The C flag in the CCR is set to zero when calculation results in a carry Since this flag is named "Carry", it is set to when calculation results in a carry Otherwise, zero 11 ( T )One address of the memory is bits Except for special microcomputers such as 4-bit types, bits (1 byte) of the memory are used per address 12 ( T )8-bit data can be stored in both even- and odd-numbered addresses Since 8-bit data exactly occupies one address of the memory, it can be stored in either an even- or odd-numbered address 13 ( T )16-bit data must be stored in an even-numbered address Since the H8/3048 reads and writes 16 bits of data at a time, the upper bits must be stored in an even-numbered address and the lower bits in the next address If 16-bit data is stored in an odd-numbered address and the next even-numbered address, reading/writing will fail http://resource.renesas.com Page 35 4.2 Instruction Configuration This section describes some basic instructions used in assembly language And the subsequent sections explain how to develop a program using them MOV instruction The MOV (MOVe data) instruction is used for data transfer Although "transfer" may sound like moving the original data, the function of this instruction is similar to copying and the original data remains It is available from the memory to a general-purpose register, from a generalpurpose register to the memory, between general-purpose registers and from data to a general-purpose register This instruction is most frequently used in a program Samples Transfers 8-bit data from the R0L to the R1L MOV.B R0L,R1L MOV.B @H'1000,R0L Transfers the bits in the H'1000 address to the R0L MOV.B R1L,@H'2000 Transfers the R1L to the bits in the H'2000 address MOV.B #1,R0L Inputs (transfers) data "1" in the R0L ADD instruction The ADD (ADD binary) instruction is used for addition The results are stored in the general-purpose register written on the right Samples Adds the R1L and R0L and stores the results in the R1L ADD.B R0L,R1L ADD.B #H'12,R0L Adds the R0L and H'12 (18 in decimal notation) and stores the results in the R0L SUB instruction The SUB instruction (SUBtract binary) is used for subtraction It subtracts the contents of the general-purpose register written on the left from those on the right and stores the results in the register written on the right Sample SUB.B R0L,R1L Subtracts the R0L from the R1L and stores the results in the R1L CMP instruction The CMP (CoMPare) instruction is used for comparison It performs subtraction not to obtain the results but simply for comparison What matters most is not what the answer is but how N, Z, V and C in the CCR change after subtraction In other words, the CMP instruction simply performs subtraction and changes N, Z, V and C in the CCR A CMP instruction must be followed by a conditional branch instruction This is because comparison is meaningless without conditional test http://resource.renesas.com Page 36 Samples Subtracts the R0L from the R1L, changing the CCR CMP.B R0L,R1L Conditional branch instruction Subtracts H'12 (18 in decimal notation) from the R0L, CMP.B #H'12,R0L changing the CCR Conditional branch instruction BRA instruction The BRA (BRanch Always) instruction is called "unconditional branch instruction" Executing this instruction results in branching to the specified address Branching is similar to "jumping" It causes jumping forward or backward, skipping some instructions The destination address is specified by giving it a name ("symbol") Sample BRA ABC Unconditionally branches to the symbol ABC Instruction ABC: Instruction BGT instruction The BGT (Branch Greater Than) instruction is one type of conditional branch instruction It compares data as a "signed binary number" and branches to the specified instruction if it is greater Otherwise, it does nothing and the next instruction is executed Sample CMP.B R0L,R1L Compares the R1L with the R0L BGT ABC If the R1L is greater, branches to the symbol ABC Instruction Otherwise, the next instruction is executed Instruction ABC: Instruction BHI instruction The BHI (Branch HIgh) instruction is another type of conditional branch instruction It compares data as an "unsigned binary number" and branches to the specified instruction if it is greater Otherwise, it does nothing and the next instruction is executed Sample Compares the R1L with the R0L CMP.B R0L,R1L If the R1L is greater, branches to the symbol ABC BHI ABC Instruction Otherwise, the next instruction is executed Instruction ABC: Instruction http://resource.renesas.com Page 37 4.3 Adder Program 4.3.1 How to Develop a Source Program This section describes how to develop a source program to add 8-bit data with assembly language It is assumed that 8-bit unsigned binary numbers are stored in the H'2000 and H'2001 addresses of the memory Here, you will create a program to add these two data blocks and write the results in the H'2002 address Up to bits of results are obtained even if addition results in a carry, generating bits Since addition is performed, the following instruction is used: ADD.B R0L,R1L Any 8-bit register can be used as general-purpose registers other than the R0L or R1L To perform addition using this instruction, you must input data to be added in the R0L and R1L beforehand To input data from the H'2000 address of the memory to the R0L general-purpose register, use the following instruction: MOV.B @H'2000,R0L To input data from the H'2001 address to the R1L general-purpose register, use the following instruction: MOV.B @H'2001,R1L H'2000 represents an address in hexadecimal notation Memory addresses are generally expressed in this notation "@" is a mandatory prefix to indicate a memory address The above instructions should be arranged as follows for addition: MOV.B @H'2000,R0L MOV.B @H'2001,R1L ADD.B R0L,R1L This, however, simply stores the addition results in the R1L and they are not written in the H'2002 address of the memory MOV.B R1L, @H'2002 Use the above instruction to write the addition results in the H'2002 address of the memory MOV.B @H'2000,R1L MOV.B @H'2001,R1L ADD.B R0L,R1L MOV.B R1L,@H'2002 Consequently, addition is completed with the above four instructions Simply arranging these four instructions, however, will not make a complete program http://resource.renesas.com Page 38 After reading one instruction, the CPU automatically stores the address of the next instruction in the PC and reads the next instruction after execution is completed Since the CPU does not understand whether the next address has an instruction or not, it assumes that there must be an instruction in the next address and executes it even after executing the above four instructions This results in a runaway since the CPU executes non-existing instructions To prevent this, use the BRA instruction as follows: MOV.B @H'2000,R1L MOV.B @H'2001,R1L ADD.B R0L,R1L MOV.B R1L,@H'2002 ABC: BRA ABC The above instruction leads to unlimited execution of the BRA instruction This prevents the program from proceeding and running away The program, however, is still incomplete This program does not indicate at which address of the memory the program itself should be located It is indicated using an assembler control instruction The assembler control instruction is not executed by the CPU but used to instruct an assembler, which is machine language conversion software .SECTION PROG,CODE,LOCATE=H'1000 Use the control instruction shown above Every assembler control instruction is prefixed with "." (period) With it, you can easily distinguish between assembler control instructions and those executed by the CPU (execution instructions) .SECTION indicates the section control instruction, PROG represents the section name (section can be named originally based on certain rules), CODE refers to the instruction code, and LOCATE=H'1000 specifies that instructions should be located starting from the H'1000 address of the memory The CPU control instruction to specify the CPU type is also required since the assembler for the H8/300H is compatible with several CPU types In addition, the END control instruction must be written on the last line Finally, a complete program is written as shown in List 4.1 List 4.1: Simplest Program http://resource.renesas.com Page 39 This source program is converted into machine instructions by the assembler as follows: Address Machine instruction Instruction H'001000 H'001004 H'001008 H'00100A H'00100E 6A082000 6A092001 0889 6A892002 40FE ABC: CPU SECTION 300HA PROG,CODE,LOCATE=H'1000 MOV.B MOV.B ADD.B MOV.B BRA END @H'2000,R0L @H'2001,R1L R0L,R1L R1L,@H'2002 ABC The machine instruction is expressed in hexadecimal notation Since one address of the memory is bits, it is expressed with a 2-digit, hexadecimal number Since the machine instruction "MOV.B @H'2000,R0L" is eight digits, four addresses of the memory are used to store it (called "4-byte instruction") In the case of the H8/300H, the shortest instruction is bytes and the longest is 10 bytes The first MOV instruction is stored in the four addresses starting from H'1000 (H'001000) and the next MOV instruction in the four addresses from H'1004 The ADD instruction is stored from H'1008, the next MOV instruction from H'100A and the last BRA instruction from H'100E Since CPU, SECTION and END are control instructions, they not correspond to machine instructions Let's consider how the contents of general-purpose registers and the memory change when a program is executed It is assumed that H'4C (B'01001100) is stored in the H'2000 address and H'40 (B'01000000) in the H'2001 address Also, the contents of the R0L general-purpose register is assumed to be H'00 and those of the R1L to be H'00 [Simulation] http://resource.renesas.com Page 40 4.3.2 Rules on Source Programs There are some rules when developing source programs in assembly language If they are not followed, an error will occur on assembly This section describes the rules relating to source programs Configuration of an instruction An instruction is configured as follows: Sample instruction ADD.B R0L,R1L ADD is the operation portion of the instruction representing "ADDition" .B is the size specification portion indicating that the instruction's operation is in bit units (.W represents 16 bits and L represents 32 bits.) The R0L and R1L are collectively called "operand", which is an operation target The R0L (on the left) is specifically called the source operand And the R1L (on the right) is specifically called the destination operand The results and answers to calculations are stored in the destination operand Note that some instructions have only one operand (destination operand) or none at all Sample instruction with one operand Sample instruction with no operand INC.B R0L RTS How to write one line (without a symbol) Rules: One or more spaces or tabs must be placed at the beginning Instructions and operands must be separated by one or more spaces or tabs An instruction may be written in both upper and lower cases A line must end with a return http://resource.renesas.com Page 41 Samples: MOV.B MOV.B R0L,R1L mov.b r0l,r1l Mov.b @h'1000,R1h R0L,R1L MOV.BR0L,R1L Good sample Good sample Good sample Bad sample (no space or tab at the beginning) Bad sample (instruction and operand are not separated by one or more spaces or tabs) How to write one line (with a symbol) Rules: Write a symbol first Suffix the symbol with ":" (colon) The rest are the same for a line without a symbol Rules on symbols: Available characters are A to Z, a to z, to 9, _ and $ Upper and lower cases are handled as different characters The first character must not be a numeric value The same name as a CPU internal register must not be used Samples: Good sample LOOP: MOV.B R0L,R1L R1: MOV.B R0L,R1L Bad sample (the same name as an internal register is used as a symbol) Samples available as symbols: Loop Upper and lower cases may be mixed "_" is available as a character End_of_Loop A numeric value can be used except at the beginning DATA1 Samples not available as symbols: 1second Starts with a numeric value Second/100 "/" is used Total.Data "." is used CCR The same name as an internal register Lines with symbols only Only symbols may be written on lines In this case, they are treated like those written for the following instruction So, the two samples below represent exactly the same program: Samples: LOOP: MOV.B R0L,R1L Sample with symbol and instruction written on the same line Sample with symbol and instruction LOOP: MOV.B R0L,R1L written on different lines 4.3.3 Inserting Comments Comments are inserted to make programs readily understood They serve as memos and have no influence on program operation Comments can be http://resource.renesas.com Page 42 inserted in two ways One is to place ";" (semicolon) at the beginning of a line, which causes the entire line to be treated as a comment All characters such as alphabets, numeric values and special symbols can be used Sample: ;******************************** ;* H8/300H Sample program * ;* 2002.9.1 * ;******************************** Another is to insert a comment by suffixing an instruction with ";".This way, you can add a comment to each instruction Program operation is easy to understand when each instruction has a comment In List 4.2, comments have been added to the adder program described earlier List 4.2: Program with Comments 4.3.4 How to Use EQU Control Instruction If a memory address is written in hexadecimal notation in a program, it is difficult to determine what kind of data is included So, it is helpful if an address including data can be expressed by a symbol, rather than in hexadecimal notation The EQU control instruction is the simplest way to express an address with a symbol DATA1: EQU H'2000 DATA2: EQU H'2001 ANSWER: EQU H'2002 The above instructions make DATA1 equal to H'2000.For example, in the following instruction: MOV.B @DATA1,R0L DATA1 is converted into H'2000 by the assembler and it becomes equal to: MOV.B @H'2000,R0L http://resource.renesas.com Page 43 This method is useful when numeric values and addresses are fixed and will not be changed List 4.3 shows a program rewritten with this method List 4.3: Program Using EQU 4.3.5 How to Use RES Control Instruction The RES control instruction is used to reserve an area for writing in the RAM.A RAM address is generally specified not by the EQU control instruction but by a combination of RES and SECTION control instructions This is because the RES control instruction has the following benefits: The beginning address can be freely changed using SECTION Data areas can be easily inserted or deleted The RES control instruction is used as follows: Samples: SECTION WORK,DATA,LOCATE=H'2000 AB: RES.L ; Reserves one 32-bit area using the symbol AB ; Reserves one 8-bit area using the symbol CD CD: RES.B RES.B ; Simply reserves one 8-bit area This is used to correct the 16- or 32-bit area to be reserved next to an even-numbered address EF: RES.W XYZ: RES.B ; Reserves two 16-bit areas using the symbol EF ; Reserves six 8-bit areas using the symbol XYZ A symbol attached to a reserved area represents the address In the example below, since the WORK section is located at the H'2000 address, DATA1, DATA2 and ANSWER represent the H'2000, H'2004 and H'2006 addresses respectively .SECTION WORK,DATA,LOCATE=H'2000 DATA1: RES.L http://resource.renesas.com Page 44 DATA2: ANSWER: RES.W RES.B 1 If the RAM starts from the H'2000 address as with the program described earlier, write as follows to reserve an area for writing there: SECTION WORK,DATA,LOCATE=H'2000 DATA1: RES.B DATA2: RES.B ANSWER: RES.B In the above, DATA1, DATA2 and ANSWER represent the H'2000, H'2001 and H'2002 addresses respectively List 4.4 shows a program rewritten using RES control instructions You can see that the ROM starts from the H'1000 address, in which a program is stored, and the RAM starts from the H'2000 address, which is used as a work area http://resource.renesas.com Page 45 List 4.4: Program Using RES 4.3.6 How to Use DATA Control Instruction The programs described so far require that values to be added be written at the DATA1 and DATA2 addresses by some means before they are executed This is because DATA1 and DATA2 are stored in the RAM Since the contents of the RAM are cleared when it is turned off, it is unpredictable what are stored there after it is turned on again In other words, you cannot determine what must be included in the RAM data area In the RAM, you can only reserve an area for writing data temporarily On the contrary, the DATA control instruction is used to set a certain value in the ROM Although the use is similar to RES, it differs in that the control instruction is followed by "the value to be set in an area", not by "the count of areas" Samples: SECTION WORK,DATA,LOCATE=H'1100 ; Reserves an 8-bit area including a value "10" using the AB: DATA.B 10 symbol AB CD: DATA.B H'A6 ; Reserves an 8-bit area including a value "H'A6" using the EF: DATA.W H'12AB; Reserves a 16-bit area including a value "H'12AB" using symbol CD XYZ: DATA.L 40000 the symbol EF ; Reserves a 32-bit area including a value "40000" using the symbol XYZ In the case of the DATA control instruction, a symbol attached to a reserved area also represents the address .SECTION WORK,DATA,LOCATE=H'1100 DATA1: DATA.L 10000 http://resource.renesas.com Page 46 DATA2: ANSWER: DATA.W DATA.B 1000 10 In the above example, since the WORK section is located at the H'1100 address, DATA1, DATA2 and ANSWER represent the H'1100, H'1104 and H'1106 addresses respectively If the ROM is also located at the H'1100 address, and if "10" and "100" to be added should be provided separately, write as follows to prepare a separate section for storing the addition results in: SECTION ROM_DATA,DATA,LOCATE=H'1100 DATA1: DATA.B 10 DATA2: DATA.B 100 SECTION RAM_DATA,DATA,LOCATE=H'2000 ANSWER: RES.B The above makes DATA1 represent the H'1100 address including "10" ("H'0A" in hexadecimal notation), DATA2 represent the H'1101 address including "100" ("H'64" in hexadecimal notation) and ANSWER represent the H'2000 address List 4.5 shows a program rewritten using DATA control instructions You can see that the program starts from the H'1000 address, "10" and "100" are stored in the H'1100 and H'1101 addresses respectively as data in the ROM, and the H'2000 address is used as a work area in the RAM http://resource.renesas.com Page 47 List 4.5: Program Using DATA 4.4 Collating Sequence Test Program This section describes a program to test the collating sequence Let's assume that two 8-bit data blocks (both unsigned) are stored in the RAM and you create a program to sort them in descending order The program should also store larger data in the DATA1 (H'2000) address and smaller in the DATA2 (H'2001) As instructions to test conditions, the BHI (Branch HIgh) and BGT (Branch Greater Than) instructions have already been described The BHI tests the collating sequence assuming data to be unsigned and the BGT assuming data to be signed In combination with the CMP instruction, they are used as follows: ; Compares the contents of the R1L and R0L CMP.B R1L,R0L BHI ABC ; Branches to ABC if the contents of the R1L are greater Instruction ; Otherwise, the next instruction is executed Instruction ABC: Instruction The BHI is an instruction to perform branching if data is greater "If data is greater" means "if the data on the right is greater than that on the left based on comparison" by the CMP instruction Attach a symbol to the instruction you want to branch If the condition is satisfied, branching forward or backward occurs, skipping some instructions Otherwise, the next instruction is executed The BHI instruction performs branching when both Z and C flags in the CCR are The CMP instruction subtracts the R1L from the R0L If Z is as a http://resource.renesas.com Page 48 ... unpredictable what are stored there after it is turned on again In other words, you cannot determine what must be included in the RAM data area In the RAM, you can only reserve an area for writing data... write as follows to prepare a separate section for storing the addition results in: SECTION ROM_DATA,DATA,LOCATE=H''1100 DATA1: DATA.B 10 DATA2: DATA.B 100 SECTION RAM_DATA,DATA,LOCATE=H''2000 ANSWER:... reserve an area for writing there: SECTION WORK,DATA,LOCATE=H''2000 DATA1: RES.B DATA2: RES.B ANSWER: RES.B In the above, DATA1, DATA2 and ANSWER represent the H''2000, H''2001 and H''2002 addresses