bits instruction

This instruction is used to create bit string values. It can either take a register index and create a bit string with width specified by the integer found in the register it received, or a bit string (or binary) literal.

Bit string literals may be written as binary, octal, or hexadecimal numbers.

bits %1 local 0b10011101
bits %3 local 0o64014275
bits %2 local 0xdeadbeef
        

One caveat that must be taken into account when using literals is that their width is expanded to the nearest multiple of eigth. For example, if a literal in source code might be encoded using 13 bits it will be actually encoded on 16 bits. The expansion is performed by padding the bit string with zeroes on the left (i.e. the higher bits will be zeroes).

bits  %1 local 0b11011  ; a 5-bit wide literal
print %1 local          ; will print '00011011' (8 bits)
        

Literals are a handy syntax when a bit-pattern is known at write- or compile time, and can be used to avoid creating an empty bit strings and enabling bits one-by-one.

bitat and bitset instructions

The bitat instruction fetches value of the bit at a specified index from a bit string. It needs a target register, a bit string value, and an index:

bits   %1 local 0b10101010
istore %2 local 1
bitat  %3 local %1 local %2 local   ; from bit string in register 1, get value of the bit with the index
                                    ; specified by the integer in register 2, and
                                    ; put it in register 3
        

The bitset instruction sets value of the bit at a specified index inside a bit string, modifying the bit string in-place.

bits   %1 local 0b10101000
istore %2 local 1

bitset %1 local %2 local true   ; in bit string in register 1, set bit with index specified by the integer
                                ; in register 2 to true (enable the bit)

bitset %1 local %2 local false  ; in bit string in register 1, set bit with index specified by the integer
                                ; in register 2 to false (disable the bit)

bitset %1 local %2 local %3 local   ; in bit string in register 1, set bit with index specified by the integer
                                    ; in register 2 to the boolean value in register 3 (disabling, or enabling
                                    ; the bit depending on contents of register 3)
        

Bitwise logic

The four operations available: and, or, xor, and not are implemented by bitand, bitor, bitxor, and bitnot instructions. The first three instructions share the three-operand structure: output-lhs-rhs, while bitnot has only two operands: output and source.

bits %1 local 0b11001010
bits %2 local 0b10101100

; in all these instructions register 1 is the left-hand side of the operation, and
; register 2 is the right-hand side
bitand %3 local %1 local %2 local
bitor  %3 local %1 local %2 local
bitxor %3 local %1 local %2 local

; not registes only one source operand: just the bit string to negate
bitnot %3 local %1 local
        

Shifts

Viua provides instructions to perform arithmetic and logical shifts, in both directions (left and right). All shift instructions have the same operand structure: output-source-offset. Shifted bit strings are modified in-place.

The output operand (also known as "target register") stores the bits that were "shifted out". For example, if an 8 bit wide bit string is shifted to the left by 3 bits, the target register will store a 3 bit wide bit string.

; all instructions presented below shift the value in register 1 by the
; number of bits specified by the integer in register 2, and
; store the "shifted out" bits in register 3

shl  %3 local %1 local %2 local     ; logical shift left
shr  %3 local %1 local %2 local     ; logical shift right
ashl %3 local %1 local %2 local     ; arithmetic shift left
ashr %3 local %1 local %2 local     ; arithmetic shift right
        

It is also possible to use void target register as the output of shift instructions to discard the "shifted out" bits if they are not interesting.

shl  void %1 local %2 local
shr  void %1 local %2 local
ashl void %1 local %2 local
ashr void %1 local %2 local
        

Rotates

Viua provides instructions to perform left and right bit rotations. Rotations are performed on bit strings in-place.

rol %1 local %2 local   ; rotate, to the left, the bit string in register 1 by
                        ; number of bits specified by the integer in register 2

ror %1 local %2 local   ; rotate, to the right, the bit string in register 1 by
                        ; number of bits specified by the integer in register 2