
The JMOS family of IC's includes some serial adder MSI devices. Following are the presumed or functionally equivalent internal structure of these devices.
The TM4006 is a serial adder including a carry latch. This is standard construction for a serial adder except for the addition of the subtraction facility. When pin 1 is FALSE the B operand is subtracted from the A operand.
See the Toshiba BC1212 for an example of it's use.
TM4006 Serial Adder Functional Equivalent of Internal Structure 
The HD3112 is considerably more complex, providing BCD sum correction as well as subtraction. As shown below, it is presumed to contain two serial adders, a 4bit shift register and additional logic for subtraction, ten's carry detection and the BCD sum correction.
The first adder adds or subtracts the two input operands. For subtraction, A is subtracted from B (BA). The raw sum for a digit is collected in the 4bit shift register. At the end of the digit, as determined by the ØB8 input, the shift register state along with the bit carry is used to make the carry out of the digit into a ten's carry. The second adder performs the BCD sum correction (normalisation) by adding or subtracting 6 to the digit coming out of the shift register if a digit carry was detected.
Notice that the carry circuitry for the normalisation adder is simplified from that of the operand adder. This is possible because one input is limited to being either 0 or 6 and the carry into the 1 bit must be 0. Consequently:
The ØDE*B8 signal (DE=End Digit) is used to suppress the carry at the end of a number cycle from being fed in at the beginning of the next number cycle.
See the Commodore DAC612 or Casio 121B for examples of use of the HD3112.
HD3112 Serial BCD Adder Functional Equivalent of Internal Structure 
Several aspects of this device remain undeciphered:
JMOS Serial Adders
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2007 Jan 