The armlet processor

The data path

At this point you may want to recall the design of the data path, and in particular the armlet instructions for operating the data path from Round 4: Sequential logic, state and feedback.

The program counter and processor status

To upgrade our design to a full stored-program architecture, it will be convenient to have available two more registers.

The program counter (PC) is a register that stores the memory address from which the current instruction was loaded (or is currently being loaded, in case we are loading a 32-bit instruction and are still waiting for the immediate data to arrive from memory).

The processor status (PS) register contains status bits that are used to control program execution, in particular the processor status includes three bits that store the result of the most recent comparison instruction that was executed.

The comparison instructions

The full armlet architecture supports two comparison instructions, one that takes two register operands, and one that compares a register operand with 16 bits of immediate data:

cmp $A, $B           # compare $A (left) and $B (right)
cmp $A, I            # compare $A (left) and I (right)

The result of the comparison is stored in three flag bits in the processor status register: equal, greater, and above.

The equal flag is set to 1 if and only if the left operand equals the right operand; otherwise the flag is set to 0. The greater flag is set to 1 if and only if the left operand is greater than the right operand (both operands are viewed as signed integers); otherwise the flag is set to 0. The above flag is set to 1 if and only if the left operand is greater than the right operand (both operands are viewed as unsigned integers); otherwise the flag is set to 0.

The processor status register stores the result of the comparison until the next comparison is made, at which point the status is updated to reflect the result of the next comparison.

Default flow of execution

When instructions are loaded from memory for execution, the default is to load the next instruction from the memory address that immediately succeeds the address of the current instruction. That is, after the clock triggers, the value of the program counter is the current value of the program counter plus one.

The default flow of execution may be altered by jump instructions and branch instructions.

The jump and branch instructions

A jump instructs that the program execution is to continue at the memory address given as an operand, which may be either the contents of a register or an address supplied as immediate data. For example, jmp 12345 instructs that the next instruction must be loaded from the memory word with address 12345. In effect, a jmp is like a mov whose target is the program counter – the execution will continue from the indicated address as soon as the clock triggers.

A branch instruction is like a jump, but the jump is executed conditional to the current contents of the program status register, that is, the result of the most recent comparison instruction. For example, the branch instruction beq 12345 jumps to the address 12345 if the result of the most recent comparison was that the two operands were equal; otherwise the processor follows the default flow of execution.

Here is a summary of all the jump and branch instructions that take a register operand:

jmp $A           # jump to address $A
beq $A           # ... if left == right (in the most recent comparison)
bne $A           # ... if left != right
bgt $A           # ... if left > right  (signed)
blt $A           # ... if left < right  (signed)
bge $A           # ... if left >= right (signed)
ble $A           # ... if left <= right (signed)
bab $A           # ... if left > right  (unsigned)
bbw $A           # ... if left < right  (unsigned)
bae $A           # ... if left >= right (unsigned)
bbe $A           # ... if left <= right (unsigned)

Here is a summary of all the jump and branch instructions that take an immediate operand:

jmp I            # jump to address I
beq I            # ... if left == right (in the most recent comparison)
bne I            # ... if left != right
bgt I            # ... if left > right  (signed)
blt I            # ... if left < right  (signed)
bge I            # ... if left >= right (signed)
ble I            # ... if left <= right (signed)
bab I            # ... if left > right  (unsigned)
bbw I            # ... if left < right  (unsigned)
bae I            # ... if left >= right (unsigned)
bbe I            # ... if left <= right (unsigned)

Jumps and branches versus if-then-else, while, and such

The jump and branch instructions may appear somewhat cumbersome, but they are sufficient to implement all the functionality offered by higher-level constructs such as an if-then-else statement or a while-loop in Scala. We will get practice on this soon.

Reset

So how does the processor logic start to execute instructions, and from which memory address? Here our convention is to start from the memory address 0. The processor is instructed to start by means of a reset input. Set the reset input to true, trigger the clock, and set the reset input back to false.

When the clock triggers, all processor registers are reset to zero, and the memory interface unit issues a read at address 0. Then it is off to the races – the processor will follow the rules of execution until there is a halt.

Halt

The processor stops the execution if it encounters and unused instruction opcode or a halt instruction hlt.

hlt              # halt execution

The processor signals that a halt has occurred by giving true output to hlt_f. A halt may be reversed only by issuing a processor reset.

Trap (debugging)

For debugging purposes the processor supports a trap instruction trp that can be used to signal a break in execution so that the programmer can inspect what the processor is doing.

trp              # trap (break out of execution for debugging)

Our Ticker interface to the armlet processor will break the execution of a running program whenever a trap instruction is encountered.

The armlet architecture (*)

The complete and upgraded armlet architecture is summarized in the following diagram:

The only external inputs to the processor are the input bus read_in that receives the result of the read from memory, and the reset input reset_e for issuing a processor reset.

The external outputs of the processor are the halt indicator hlt_f and the outputs mem_read_e, mem_write_e, mem_addr, mem_data to control the memory unit.

The control and execution unit is responsible for loading and executing the program. The unit is partitioned internally into three subunits, each of which interacts with the units of the data path.

The instruction loader unit takes care of loading instruction from memory, in particular in the case when the instruction consists of multiple words (that is, has an immediate data word).

The instruction decoder unit decodes a complete instruction it receives from the instruction loader unit and configures the data path.

The jump and branch unit updates the value of the program counter and the processor status register based on their current values, the current instruction, and the result of comparing the operands in the ALU. The jump and branch unit also configures the memory interface unit to issue a memory read for the next instruction, unless the current instruction is a load (loa) or store (sto), in which case the processor must wait until the load or store completes and only then issue the read for the next instruction.

As can be read from the rough description above, the full armlet architecture is already somewhat intricate because of the necessary choreography between units to access the memory.

Yet it is all just sequential logic. In fact, the complete armlet processor is exactly 5574 Gate-objects in minilog!

In short, sequential logic is all one needs to build a programmable computer from scratch. In our case it took less than 500 lines of Scala code in package armlet to build a simple processor architecture.