General

The Wombat5w is an expansion of Dale Skrien's Wombat5 machine. It adds more memory and widens the instructions from two bytes to three bytes to allow for 16 bit addresses. It also widens the values from 16 bits to 32 bits to allow for full Java ints. It adds a separate heap memory and some machine instructions to deal with the heap.

Since the machine is implemented virtually in Dale Skrien's CPUSim, this description is broken down by the components needed to define the machine in CPUSim.

Hardware Components

Registers

The machine uses these individual registers internally to do its computations and manage its state. They are not accessible to the assembly language program directly.

The ir (instruction register) is essentially the memory data register for the Main RAM (instructions).

I'll have to analyze what the frames look like.

There is no top register since register A7 now plays the role of the stack pointer.

Register arrays

There is one register array called A, with 8 general purpose registers, A0 through A7. Each is 32 bits wide. Register A7 now plays the role of the stack pointer. We shall use A6 as the frame pointer.

These registers are addressed by 3 bits in an instruction.

Condition bits

The only condition bit used is the halt bit (bit 0) of the status register, which the stop instruction sets to true. Arithmetic instructions also use it as their overflow bit, which stops the machine on an arithmetic overflow. Assembly language programs do not deal with this condition bit directly.

RAMs

There are three RAM areas. The sizes are in bytes.

All of these have size 65536 bytes, addressable with 16 bits. Since each data word has 4 bytes, the Stack and Heap RAMs can each hold 16384 words. Since each instruction has 3 bytes, the Main RAM can hold 21845 instructions, with one byte left over. This should be enough for our test programs.

Microinstructions

The microinstructions are used to implement the machine instructions. A detailed understanding of them is not necessary to compile programs to the machine language.

EQUs

The CPUSim assembly language allows the use of name equivalences to represent numeric constants in a more understandable symbolic form. The Wombat4w defines equivalences for the array registers, allowing assembly code to represent them as A0 through A7 instead of 0 through 7.

The assembly language program can also define its own local EQUs, if desired.

Fetch sequence

The fetch sequence uses microinstructions to load and decode (run) the next instruction. The assembly language program is not concerned directly with it.

Machine instructions

A negative field size indicates a "don't care" field.

Note that the Wombat5w alloc instruction reverses the roles of the operands from the Wombat4w alloc.

Previous version of loadi and storei

A previous version of this machine did not have a fixed offset for the loadi and storei instructions. The instructions then looked like this.

Corrections

Corrections that need to be made:

• Change microinstruction ir(11-23)->mar to ir(11-23)->mar(3-15)
• Change mdr->ir to mdr(0-23)->ir (name change only)
• Change mdr->pc to mdr(16-31)->pc (name and start bit)
• Change pc->mdr to pc->mdr(16-31)
• In fetch sequence, change mdr->ir to mdr(0-23)->ir (above change should fix this)
• Add clear-mar to load and store
• Change long opcodes to negative operand sizes
• Review loads and stores

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