Signals and Types

Understanding RHDL’s type system — how signals are declared, sized, and connected.

Signal Declaration

Signals in RHDL are typed by their bit width:

input  :data, width: 8    # 8-bit input
output :result, width: 16  # 16-bit output
signal :temp, width: 32    # 32-bit internal signal

Single-bit signals omit the width:

input  :clk              # 1-bit clock
output :valid             # 1-bit flag

Access individual bits or ranges:

data[0]       # LSB
data[7]       # MSB
data[3..0]    # lower nibble
data[7..4]    # upper nibble

Combine signals with array syntax:

result <= [upper_byte, lower_byte]

Use Ruby integers directly. RHDL infers the required width:

output <= 0xFF
output <= 0b1010
output <= 42

Signals are automatically extended or truncated to match the target width on assignment. Explicit casting is available when you need control:

narrow <= wide.truncate(8)
wide   <= narrow.extend(16)
wide   <= narrow.sign_extend(16)