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Hestia[1] is a framework for defining ISAs. It provides a Ruby-based `DSL' to define an ISA, and from it can assemble and disassemble the specified assembly operations, as well as simulate both assembly files and already-assembled binary files.

Defining an ISA

Defining an ISA can be done anywhere, though giving it its own file is probably a good plan. However, in this case, for now, you will need some external way to access the defined ISA object. A sample ISA definition is provided in the isa_def.rb file that stores the resulting ISA in an @isa3220 instance variable.

Defining an ISA starts at the isa method. It only takes a block, within whose context you can go about specifying the ISA definition.


Each ISA has several directives available to it to set up the ISA's characteristics. These set up instruction widths, data widths, etc. These can be identified as the instance methods available in the ISA class. They are:


The width of a single instruction, in bits.


The width of a word of data, in bits.


The byte ordering in memory. :little_endian and :big_endian are

the valid values.

The start address of the instruction part of memory.


Any method that ends in _width sets the width of a particular

field in an assembled binary instruction, in bits.[2]

This is a special width call that sets the width for all

register fields in binary instructions (see below).

The binary representation of an instruction that,

when detected, will cause simulations to terminate.

Register Setup

There are also a few register-related declarations that can be made:


Takes a register number and an :as option to specify a

name that the register can be identified by in op definitions. This causes
that register number to freeze its value to 0. Any writes to it will not
change its value, and will emit a warning.

Takes a register number and an :as option to specify a

nam ethat the register can be identified by in op definitions. This causes
that register number to emit warnings when it is written to. This is more of
a debugging tool, since the register's value is still modifiable.

Specifies a list of valid formats for specifying a register in

assembly instructions. +:val+ indicates the actual register number. For
example, 'R:val' means that `R13' will translate to register to register
number 13.

Specifies parameters in ops that are considered register

parameters. In string patterns for assembly ops, these are parsed using one
of the register formats set via +reg_formats+. In patterns for binary ops,
these fields are given the width specified by +reg_width+.

Specifies the register parameter that, when it

appears in an instruction, will have the result of the op's execution block
(see below) stored into it.


Finally, the most important part of the ISA is the definition of actual operations that the ISA accepts. These come in two forms: regular ops and mnemonics. Mnemonics expand into several base instructions. Ops can be grouped into `op types', which are groups of operations that share properties, including binary and assembly representations.

Here is an example of a simple operation:

op('mib0', :with_opcode => 12, :takes => ':op :dest, :imm',
                               :produces => ':opcode:dest:imm') do
  regs[dest][31..8] | imm

This operation is has the name `mib0'. This is the name that will be used for it in an assembly file.

The :takes option specifies the format of the op in an assembly file. The :op placeholder is always the name of the operation. :dest is a register, as specified via the register_args method. It is also the destination register, as specified via the destination_register method. :imm, because it was not listed via register_args beforehand, is just taken to be an immediate value.

The :produces option specifies the format of the op in binary form. Each value in this is a number, and the widths of each of these is specified via the +*_width+ methods before ops are defined. Values can be fixed by passing the :with_* options. Each of these fixes a particular placeholder in the binary form to a specific value. These fixed values are used for disassembling and for assembling. When disassembling, the non-fixed placeholders are assigned as local variables. When assembling, these same placeholders have their values taken from existing local variables, typically parsed from the :takes option.

Finally, the op takes a block. This block executes the actual operation. It has access to a few things:

  • All variables specified as placeholders in :takes and :produces.

  • A regs collection that represents all the registers in the ISA.

  • A mem collection that represents all the memory in the ISA.

If the op has a destination register (specified by destination_register) as one of its placeholders, then the result of the block is assigned into that register once the block is done executing. Otherwise, the result of the block is discarded. In the above example, the block reads the destination register's top 24 bits, then uses the immediate value for the bottom 8. This is the result of the block, which is then assigned back into the destination register.

Here is an exampl eof op types:

op_type 'arith', :takes    => ':op :dest, :src',
                 :produces => ':opcode:dest:src:ext',
                 :with_opcode => 0 do
  op('add',  :with_ext => 0)  { regs[dest] + regs[src] }
  op('sub',  :with_ext => 1)  { regs[dest] - regs[src] }
  op('mul',  :with_ext => 2)  { regs[dest] * regs[src] }
  op('neg',  :with_ext => 3)  { - regs[src] }

In this example, we set up an op type. Op types essentially take the same options as ops, and then all ops within them have, as their options, the options passed to the op_type combined with those passed to the op itself. In the above example, the 'add' op has the :takes, :produces, and :with_opcode options, as well as the :with_ext option. In this context the width of the ext field is set up by calling ext_width before the op type is set up. As before, the presence of the :dest parameter means that the results of each op's block gets stored into the destination register.

More examples can be observed, as mentioned above, by looking at the isa_def.rb file in the root directory of Hestia.

Assembling, Disassembling, and Simulating

To come.


<sup>1</sup>- Hestia is the Greek goddess of architecture, as it were. <sup>2</sup>- Hestia needs to know the width of at least all but one of the

fields in any given binary representation of an instruction.
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