Signetics 2650 & 2636 programming/2650 processor

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Signetics 2650A die

The 2650 was first released by Signetics in 1975. These video game consoles use the 2650A, a redesigned version that was smaller, cheaper to manufacture and had improved operating margins. The 2650A was released in 1977. A later version, the 2650B, had a few design changes and additional features, but we aren't concerned with those here. The 2650 was fabricated in NMOS, was TTL compatible, and its many control signals made it simple to interface to. It was described by Adam Osborne as being a very minicomputer-like device having been closely based on the IBM 1130. It has a number of distinguishing features:

• 32k byte address range, organised as four 8k byte pages.
• Variable-length instructions of 1, 2 or 3 bytes.
• Single bit I/O path (flag and sense pins).
• Multiple addressing modes including indirect and indexed.
• Vectored interrupt.
• Eight-level return address stack.
• Seven 8-bit registers.
• Two program status bytes.
• Powerful instruction set.

Registers

The 2650 has seven general purpose 8-bit registers. R0 is always available, whereas there are two banks of R1, R2 and R3 which are selected by the RS bit in the Program Status Word.

Program Status Word

The Program Status Word (PSW) is a special purpose register that contains status and control bits. It is divided into two bytes, the Program Status Upper (PSU) and Program Status Lower (PSL). The PSW bits may be tested, loaded, stored, preset or cleared using the instructions tpsu, tpsl, lpsu, lpsl, spsu, spsl, ppsu, ppsl, cpsu or cpsl.

PSU    7   6    5   4   3   2   1   0   S     F     II     -     -   SP2 SP1 SP0		PSL    7   6    5   4   3   2   1   0 CC1 CC0  IDC  RS   WC  OVF COM   C
S Sense input pin Pin 1 of the 2650, connected to the Vertical Reset signal. F Flag output pin Pin 40 of the 2650, determines if vertical or horizontal joystick potentiometers are read. II Interrupt Inhibit Inhibits recognition of interrupt signals. SP2,SP1,SP0 Stack Pointer Pointer to the internal Return Address Stack of 8 words. CC1,CC0 Condition code Set by most operations except branches. IDC Inter Digit Carry Stores the bit 3 to bit 4 carry in arithmetic and rotate instructions. RS Register bank Select Determines which of the two banks of registers (r1,r2,r3) is being used. WC With/without Carry Determines if the carry bit is used in arithmetic and rotate instructions. OVF Overflow Set if a two's complement overflow occurs. COM Logical/arithmetic Compare Determines how comparisons are made. C Carry Stores any carry from the high order bit during arithmetic and rotate instructions.

Reset

When a Reset pulse is applied to the 2650A, program execution is forced to begin at memory address 0. The only other thing we know for sure about the state of the processor after reset is that the Interrupt Inhibit bit in the Program Status Word is cleared. As we very much don’t want spontaneous interrupts to happen, one of the first things our program should do is set it to disable them until we ready for them. The “Signetics 2650 Initialization Application Memo” also suggests that we may set the Stack Pointer to zero, and that we should set the Register Select bit to a known state.

The With Carry bit and the Compare bit may be set to whatever condition you will need them in first. In a simple program which has them in the same state throughout, this is a straightforward decision. In more complex programs it is generally best to set them to the most used condition initially, and then switch them back and forth whenever the opposite condition is needed. If the With Carry bit is on, then the Carry bit should be initialised appropriately before the first arithmetic instruction. Otherwise there is no need to initialise Carry or any of the other bits (Condition Code, Interdigit Carry, or Overflow).

Interrupts

Certain events occurring in the PVI cause it to signal the 2650 that it needs attention by setting the interrupt line low. If the Interrupt Inhibit bit in the Program Status Word is not set, (in other words interrupts are enabled), the processor will finish the current instruction and then set the Interrupt Inhibit bit. The PVI then outputs its interrupt vector, 3, to the data bus and the processor executes a ZBSR (branch to subroutine on page zero) to address $0003.

When the interrupt service subroutine has finished, it should be exited with either a RETC or RETE instruction. The latter will re-enable interrupts.

Instruction set

All mnemonics in the 2650A instruction set are 3 or 4 characters long. The first two or three characters specify the operation, and if present, a final character indicates an addressing mode to use (see the next section, Addressing modes for more detail). Assembly code for the 2650 generally looks something like this:

loop:   lodi,r0 $20          ; load r0 from the byte immediately following, $20
        stra,r0 $1F40	     ; store r0 in the absolute address $1F40
        eorz    r0           ; exclusive-or r0 with itself
        bctr,un loop         ; branch relative, unconditionally, to address specified by 'loop'

There are 34 basic instructions:

2650A basic instruction set

Transfer	LOD	Z,I,R,A	Load register
	STR	Z,R,A	Store register
Arithmetic	ADD	Z,I,R,A	Add to register
	SUB	Z,I,R,A	Subtract from register
	DAR		Decimal adjust register
Logical	AND	Z,I,R,A	Bitwise AND to register
	IOR	Z,I,R,A	Bitwise OR to register
	EOR	Z,I,R,A	Bitwise EXCLUSIVE-OR to register
Test	COM	Z,I,R,A	Compare value to register
	TM	I	Test masked bits of register
Rotate	RRR		Rotate register right by one bit
	RRL		Rotate register left by one bit
Branch	BCT	R,A	Branch on condition true
	BCF	R,A	Branch on condition false
	BRN	R,A	Branch if register not-zero
	BIR	R,A	Increment the register and branch if it is not zero
	BDR	R,A	Decrement the register and branch if it is not zero
	ZBR	R	Branch, relative to address $0000
	BX	A	Branch, indexed
Call / Return	BST	R,A	Branch to subroutine on condition true
	BSF	R,A	Branch to subroutine on condition false
	BSN	R,A	Branch to subroutine if register not-zero
	ZBS	R	Branch to subroutine, relative to address $0000
	BSX	A	Branch to subroutine, indexed
	RET	C,E	Conditionally return from subroutine. RETE also enables interrupts
Input / Output	WRT	C,D,E	Input/output hardware is not implemented on these consoles
	RED	C,D,E
Miscellaneous	HALT		Enter WAIT state until processor is reset or interrupted
	NOP		Do nothing except fetch and execute this instruction
Program status word	LPS	U,L	Load the PSW
	SPS	U,L	Store the PSW
	CPS	U,L	Clear specified bits in the the PSW
	PPS	U,L	Preset specified bits in the the PSW
	TPS	U,L	Test specified bits in the the PSW

Addressing modes

The Signetics 2650 has four main addressing modes: Register, Immediate, Relative and Absolute. In addition Indirect and Indexing may be combined with some of these.

Register addressing

All register-to-register instructions are 1 byte in length and use R0 as the first operand, while the other can be any of R0, R1, R2, R3.

        lodz    r3          ; load r0 from r3
        addz    r2          ; r0 = r0 + r2
        lodz    r0          ; not very useful!
        eorz    r0          ; exclusive-or r0 with itself
                            ; very useful! A one byte instruction for r0 = 0

Immediate addressing

In immediate addressing the first operand, which can be any register, is specified next to the instruction; followed by the value of the other operand, which implicitly must be a constant. All immediate addressing instructions are two bytes in length.

        lodi,r2 $20        ; r2 = 32
        subi,r0 8	   ; r0 = r0 - 8
        iori,r1 %00100000  ; set bit5 of r1
        comi,r3 15         ; compare r0 to 15

Absolute addressing

In absolute addressing the first operand, which can be any register^{(but see exception in Indexed addressing below)}, is specified next to the instruction. The second argument designates the absolute address of the memory location where the other operand is located.

Relative addressing

In relative addressing the first operand, which can be any register, is specified next to the instruction. The second argument designates a memory location where the other operand is located. This second argument is a relative displacement from the current address and can be any value from -64 to +63. In practice we don't specify this value as a number. It generally takes the form of a label and the assembler does the calculation for us. If the label is too far away the assembler will generate an error message. Relative addressing is also used in branch instructions to jump to a memory location close by.

        lodr,r2 number     ; r2 = 42
        bctr,un next       ; branch to next
number: db      42         ; define a data byte = 42
next:   addi,r2 53

Indexed addressing

When indexed addressing is used, the effective address is calculated by adding the contents of the specified index register to the address field. The addition uses the value in the index register as an 8-bit positive number.

        lodi,r2 5
        loda,r0 $1F00,r2    ; r0 = contents of address $1F05

Note that the first argument is always r0. Some assemblers may allow this to be omitted from the code, but for the sake of readability it should be included if possible.

Two other options allow for the index register to be auto-decremented or auto-incremented before it is added to the base address:

        lodi,r2 5
        loda,r0 $1F00,r2+    ; r0 = contents of address $1F06
        lodi,r3 $A6
        loda,r0 $1000,r3-    ; r0 = contents of address $10A5

Indexing may only be used with absolute addressing. It may not be used with branch instructions, but two special instructions exist for this purpose. They are BXA (branch indexed, absolute) and BSXA (branch to subroutine indexed, absolute). Auto-increment and auto-decrement cannot be used with them, and register R3 must be specified as the index register.

Indirect addressing

Indirect addressing means that the argument address of an instruction is not specified by the instruction itself, but rather the argument address will be found in the two bytes pointed to by the instruction. The indirect addressing mode is indicated by an asterisk.

pointer dw $0F00            ; in most cases this would be an address in RAM, i.e. a variable
        .....
        loda,r1 *pointer    ; r1 = contents of $0F00

Indirection may be used with relative addressing as well, but since code in these consoles is operating out of ROM, and RAM addresses are generally too far away for a relative address, there is no apparent use. One exception does occur when space for code is at a premium:

              :bar   equ     $1F0E
0000  066A    :      lodi,r2 $6A
0002  CE1F0E  :      stra,r2 bar
0005  0D1F0E  :foo   loda,r1 bar
0008  0BFC    :      lodr,r3 *foo+1   ; load r3 indirectly from $0006, i.e. $1F0E

The last two instructions both load a register from bar, but the last instruction only uses two bytes rather than three.

Indexed Indirect addressing

When indexing and indirect are used together, the value of the index register is added to the indirect address, not to the value in the address field of the instruction.

Further reading

For more detailed information consult the Signetics 2650 User Manual, available as a full pdf or abridged for programmers as html.