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==Introduction==
==Introduction==
This introduction for PowerPC assembler assumes that you are somewhat familiar with the Intel assembler. It is not written as a tutorial for beginners in assembly programming. Hopefully it is possible to understand this tutorial if you just have programed in C before. This tutorial will allow you to write applications in PowerPC assembler. Disassembling compiled code is not covered, however, knowing assembler is a prerequisite to disassemble code.
This introduction for PowerPC assembler assumes that you are somewhat familiar with the Intel assembler. It is not written as a tutorial for beginners in assembly programming. Hopefully it is possible to understand this tutorial if you just have programed in C before. This tutorial will allow you to write applications in PowerPC assembler. Disassembling compiled code is not covered, however, knowing assembler is a prerequisite to disassemble code.
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The 32-bit version of the PowerPC supports the following data sizes:
The 32-bit version of the PowerPC supports the following data sizes:
−<tt>
:Byte - 8 bits
:Byte - 8 bits
:Halfword - 16 bits
:Halfword - 16 bits
:Word - 32 bits
:Word - 32 bits
−</tt>
An integer value of 12 can also be specified as 0x0C in hexadecimal or01100 in binary.
An integer value of 12 can also be specified as 0x0C in hexadecimal or01100 in binary.
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Here are examples how to define variables. The name of the variable is always set as a label followed by a colon.
Here are examples how to define variables. The name of the variable is always set as a label followed by a colon.
−<tt>
:bytevar: .byte 0 #length of one byte - init zero
:bytevar: .byte 0 #length of one byte - init zero
:shortvar: .short 0 #length of two byte - init zero
:shortvar: .short 0 #length of two byte - init zero
:wordvar: .long 0 #length of four byte - init zero
:wordvar: .long 0 #length of four byte - init zero
−<br />
:fivebytevar: .byte 11,12,13,14,15 #an array of five variables of one byte each
:fivebytevar: .byte 11,12,13,14,15 #an array of five variables of one byte each
:endof_fivebytevar: #specifies the address immediately following the array
:endof_fivebytevar: #specifies the address immediately following the array
−<br />
:stringvar: .string "Hello\n" #string variable - init to "Hello" plus newline
:stringvar: .string "Hello\n" #string variable - init to "Hello" plus newline
:: .size stringvarlen, .-stringvar #length of stringvar
:: .size stringvarlen, .-stringvar #length of stringvar
−</tt>
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The PowerPC uses fixed-length 32-bit instructions. As an example for an addi instruction this 32-bit integer is divided in the following fields:
The PowerPC uses fixed-length 32-bit instructions. As an example for an addi instruction this 32-bit integer is divided in the following fields:
−<tt>
:Opcode: 6 bits
:Opcode: 6 bits
:Source register: 5 bits
:Source register: 5 bits
:Destination register: 5 bits
:Destination register: 5 bits
:Immediate value: 16 bits
:Immediate value: 16 bits
−</tt>
So to fill a 32bit register with an immediate value you have to use two instructions moving 16 bits each. In case of a 64bit processor you need even more instructions since you have to shift the bits here too.
So to fill a 32bit register with an immediate value you have to use two instructions moving 16 bits each. In case of a 64bit processor you need even more instructions since you have to shift the bits here too.
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This will compare the signed contents of the GPR3 and GPR4 registers and set the CR7 field of the CR register accordingly. The second parameter has to be set to zero for 32bit processors.
This will compare the signed contents of the GPR3 and GPR4 registers and set the CR7 field of the CR register accordingly. The second parameter has to be set to zero for 32bit processors.
−Ifwiki>rA<rB then bit 0 of CR7 will be set. If rA>rB</nowiki> then bit 1 of CR7 will be set. If rA=rB then bit 2 of CR7 will be set.
+Ifwiki>rA<rB then bit 0 of CR7 will be set. If rA>rB</nowiki> then bit 1 of CR7 will be set. If rA=rB then bit 2 of CR7 will be set.
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This will compare the signed contents of the GPR3 register with the value 4 and set the CR7 field of the CR register accordingly. The second parameter has to be set to zero for 32bit processors.
This will compare the signed contents of the GPR3 register with the value 4 and set the CR7 field of the CR register accordingly. The second parameter has to be set to zero for 32bit processors.
−If rA<4 then bit 0 of CR7 will be set. If rA>4 then bit 1 of CR7 will be set. If rA=4 then bit 2 of CR7 will be set.
+If rA<4 then bit 0 of CR7 will be set. If rA>4 then bit 1 of CR7 will be set. If rA=4 then bit 2 of CR7 will be set.
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SLWI - Shift Left Word Immediate
SLWI - Shift Left Word Immediate
−Syntax: slwi rD,rA,SIMM (SIMM<32)
+Syntax: slwi rD,rA,SIMM (SIMM<32)
This is equivalent to: rlwinm rA,rS,n,0,31–n
This is equivalent to: rlwinm rA,rS,n,0,31–n
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SRWI - Shift Right Word Immediate
SRWI - Shift Right Word Immediate
−Syntax: srwi rD,rA,SIMM (SIMM<32)
+Syntax: srwi rD,rA,SIMM (SIMM<32)
This is equivalent to: rlwinm rA,rS,32 – n,n,31
This is equivalent to: rlwinm rA,rS,32 – n,n,31
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Example:
Example:
−<tt>
:li 4,100
:li 4,100
:mtctr 4
:mtctr 4
−</tt>
The count register is set to 100 via the GPR4. You cannot load an immediate value into the CTR.
The count register is set to 100 via the GPR4. You cannot load an immediate value into the CTR.
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Following a cmpi 7,0,3,5 instruction which compares the value in GPR3 with the integer 5 and places the resulting flags in the CR7 field of the CR register, the following conditional branch instructions can be executed. If the condition is true the program will continue at "testlabel" within the code segment. Otherwise it will just continue.
Following a cmpi 7,0,3,5 instruction which compares the value in GPR3 with the integer 5 and places the resulting flags in the CR7 field of the CR register, the following conditional branch instructions can be executed. If the condition is true the program will continue at "testlabel" within the code segment. Otherwise it will just continue.
−<tt>
:beq - branch if equal | example: beq 7,testlabel
:beq - branch if equal | example: beq 7,testlabel
:bne - branch if not equal | example: bne 7,testlabel
:bne - branch if not equal | example: bne 7,testlabel
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:ble - branch if less or equal | example: ble 7,testlabel
:ble - branch if less or equal | example: ble 7,testlabel
:bge - branch if greater of equal | example: bge 7,testlabel
:bge - branch if greater of equal | example: bge 7,testlabel
−</tt>
Example:
Example:
−<tt>
::cmpwi 4,100 /* Compare value in GPR4 with 100 */
::cmpwi 4,100 /* Compare value in GPR4 with 100 */
::bne else_label /*if not 100 goto else */
::bne else_label /*if not 100 goto else */
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:::...else statements...
:::...else statements...
:endif_label:
:endif_label:
−</tt>
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Example:
Example:
−<tt>
::li 4, 100
::li 4, 100
::mtctr 4
::mtctr 4
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::...some statements...
::...some statements...
::bdnz looplabel
::bdnz looplabel
−</tt>
In this example the count register (CTR) is loaded with the value 100 first via GPR4. Then the bdnz instruction will decrement the CTR register and branch to looplabel as long as the CTR (count register) is not zero.
In this example the count register (CTR) is loaded with the value 100 first via GPR4. Then the bdnz instruction will decrement the CTR register and branch to looplabel as long as the CTR (count register) is not zero.
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Example:
Example:
−<tt>
::li 4, 100
::li 4, 100
::mtctr 4
::mtctr 4
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::cmpwi 5,10
::cmpwi 5,10
::bdnzt eq,looplabel
::bdnzt eq,looplabel
−</tt>
In this example the count register (CTR) is loaded with the value 100 first via GPR4. Then the bdnzt instruction will decrement the CTR register and branch to looplabel as long as the CTR (count register) is not zero. However, the instruction will also test if the condition is TRUE. So if the cmpwi instruction has determined that GPR5 has the value 10, the loop will be terminated before CTR has reached zero.
In this example the count register (CTR) is loaded with the value 100 first via GPR4. Then the bdnzt instruction will decrement the CTR register and branch to looplabel as long as the CTR (count register) is not zero. However, the instruction will also test if the condition is TRUE. So if the cmpwi instruction has determined that GPR5 has the value 10, the loop will be terminated before CTR has reached zero.