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C    Basic Machine Definition

The assembly-language instructions described in this book are a superset of the actual machine-code instructions. Generally, the assembly-language instructions match the machine-code instructions; however, in some cases the assembly-language instructions are macros that generate more than one machine-code instruction (the division instructions in assembly language are examples). This appendix describes the assembly-language instructions that generate more than one machine-code instruction.

You can, in most instances, consider the assembly-language instructions as machine-code instructions; however, for routines that require tight coding for performance reasons, you must be aware of the assembly-language instructions that generate more than one machine-code instruction.

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C.1    Implicit Register Use

Register $28 ($at) is reserved as a temporary register for use by the assembler.

Some assembly-language instructions require additional temporary registers. For these instructions, the assembler uses one or more of the general-purpose temporary registers (t0 - t12). The following table lists the instructions that require additional temporary registers and the specific registers that they use:
Instruction Registers Used
ldb AT,t9
ldbu AT,t9[Table Note 1]
ldw AT,t9
ldwu AT,t9[Table Note 0]
stb AT,t9,t10[Table Note 0]
stw AT,t9,t10[Table Note 0]
ustw AT,t9,t10,t11,t12
ustl AT,t9,t10,t11,t12
ustq AT,t9,t10,t11,t12
uldw AT,t9,t10
uldwu AT,t9,t10
uldl AT,t9,t10
uldq AT,t9,t10
divl AT,t9,t10,t11,t12
divq AT,t9,t10,t11,t12
divlu AT,t9,t10,t11,t12
divqu AT,t9,t10,t11,t12
reml AT,t9,t10,t11,t12
remq AT,t9,t10,t11,t12
remlu AT,t9,t10,t11,t12
remqu AT,t9,t10,t11,t12

Table Notes:

  1. Use of registers depends on the setting of the .arch directive or the -arch flag on the cc command line.

The registers that equate to the software names (from regdef.h) in the preceding table are as follows:
Software Register
AT $28 or $at
t9 $23
t10 $24
t11 $25
t12 or pv $27


The div and rem instructions destroy the contents of t12 only if the third operand is a register other than t12. See Section C.5 for more details.

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C.2    Addresses

If you use an address as an operand and it references a data item that does not have an absolute address in the range -32768 to 32767, the assembler may generate a machine-code instruction to load the address of the data (from the literal address section) into $at.

The assembler's ldgp (load global pointer) instruction generates an lda and ldah instruction. The assembler requires the ldgp instruction because ldgp couples relocation information with the instruction.

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C.3    Immediate Values

If you use an immediate value as an operand and the immediate value falls outside the range -32768 to 32767 for the ldil and ldiq instructions or the range 0 - 255 for other instructions, multiple machine instructions are generated to load the immediate value into the destination register or $at.

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C.4    Load and Store Instructions

On most processors that implement the Alpha architecture, loading and storing unaligned data or data less than 32 bits is done with multiple machine-code instructions. Except on EV56 Alpha processors, the following assembler instructions generate multiple machine-code instructions:

Signed loads may require one more instruction than an unsigned load.

On EV56 Alpha processors, the following instructions from the preceding list generate a single instruction:

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C.5    Integer Arithmetic Instructions

Multiply operations using constant powers of two are turned into sll or scaled add instructions.

There are no machine instructions for performing integer division (divl, divlu, divq, and divqu) or remainder operations (reml, remlu, remq, and remqu). The machine instructions generated for these assembler instructions depend on the operands specified on the instructions.

Division and remainder operations involving constant values are replaced by an instruction sequence that depends on the data type of the numerator and the value of the constant.

Division and remainder operations involving nonconstant values are replaced with a procedure call to a library routine to perform the operation. The library routines are in the C run-time library (libc). The library routines use a nonstandard parameter passing mechanism. The first operand is passed in register t10 and the second operand is passed in t11. The result is returned in t12. If the operands specified are other than those just described, the assembler moves them to the correct registers. The library routines expect the return address in t9; therefore, a routine that uses divide instructions does not need to save register ra just because it uses divide instructions.

The absl and absq (absolute value) instructions generate two machine instructions.

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C.6    Floating-Point Load Immediate Instructions

There are no floating-point instructions that accept an immediate value (except for 0.0). Whenever the assembler encounters a floating-point load immediate instruction, the immediate value is stored in the data section and a load instruction is generated to load the value.

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C.7    One-to-One Instruction Mappings

Some assembler instructions generate single machine instructions. Such assembler instructions are sometimes referred to as pseudo-instructions. The following table lists these assembler instructions and their equivalent machine instructions:

Assembler Instruction   Machine Instruction  
andnot $rx,$ry,$rz bic $rx,$ry,$rz
clr $rx bis $31,$31,$rx
fabs $fx,$fy cpys $f31,$fx,$fy
fclr $fx cpys $f31,$f31,$fx
fmov $fx,$fy cpys $fx,$fx,$fy
fneg $fx,$fy cpysn $fx,$fx,$fy
fnop   cpys $f31,$f31,$f31
mov $rx,$ry bis $rx,$rx,$ry
mov val_immed,$rx bis $31,val_immed,$rx
negf $fx,$fy subf $f31,$fx,$fy
negfs $fx,$fy subfs $f31,$fx,$fy
negg $fx,$fy subg $f31,$fx,$fy
neggs $fx,$fy subgs $f31,$fx,$fy
negl $rx,$ry subl $31,$rx,$ry
neglv $rx,$ry sublv $31,$rx,$ry
negq $rx,$ry subq $31,$rx,$ry
negqv $rx,$ry subqv $31,$rx,$ry
negs $fx,$fy subs $f31,$fx,$fy
negssu $fx,$fy subssu $f31,$fx,$fy
negt $fx,$fy subt $f31,$fx,$fy
negtsu $fx,$fy subtsu $f31,$fx,$fy
nop   bis $31,$31,$31
not $rx,$ry ornot $31,$rx,$ry
or $rx,$ry,$rz bis $rx,$ry,$rz
sextl $rx,$ry addl $rx,0,$ry
unop   ldq_u $31,0($sp)
xornot $rx,$ry,$rz eqv $rx,$ry,$rz