; ram_add.asm
; add two BCD numbers on the Intel 4004
init:
 FIM R0R1, 0 ; ram 0, bank 0, address 0
 FIM R2R3, 1 ; ram 0, bank 1, address 0
 FIM R4R5, 2 ; ram 0, bank 2, address 0
 LDM 1
 SRC 0       ; select register 0, address 0
 WRM         ; store 1
 INC R1
 LDM 2
 SRC 0       ; select register 0, address 1
 WRM         ; store 2
 INC R1
 LDM 3
 SRC 0       ; select register 0, address 2
 WRM         ; store 3
 WR0         ; store length 3
 LDM 1
 SRC 1       ; select register 1, address 0
 WRM         ; store 1
 INC R3
 LDM 1
 SRC 1       ; select register 1, address 1
 WRM         ; store 1
 INC R3
 LDM 7
 SRC 1       ; select register 1, address 2
 WRM         ; store 7
 INC R3
 LDM 4
 src 1       ; select register 1, address 3
 WRM         ; store 4
 WR0         ; store length 4
begin:
 SRC 1       ; number 2
 RD0         ; read length
 XCH R6      
 SRC 0       ; number 1
 RD0         ; read length
 SUB R6
 JCN NC,len2 ; number 2 >= 1
 JUN go
len2:
 SRC 1

go:  
 RD0         ; read length
 XCH R6      ; store length
 LDM 0       ; reset addresses
 XCH R1
 LDM 0
 XCH R3

doAdd:
 JMS ldCarry
 SRC 0
 RDM         ; read digit of number 1 
 SRC 1
 ADM         ; add digit of number 2
 DAA
 SRC 2
 WRM         ; write digit of result
 TCC         ; store carry
 XCH R7
 LD R6       ; check length
 DAC
 JCN Z, endAdd ; no digits left, ready
 XCH R6
 INC R1      ; next addresses
 INC R3
 INC R5
 JUN doAdd   ; add next digits

endAdd:
 JMS ldCarry
 JCN NC, len ; carry?
 INC R5      
 LDM 1
 SRC 2
 WRM         ; store carry in next digit

len:
 LD R5       
 IAC
 WR0         ; write lenght of result

done:
 JUN done    ; end

ldCarry:
 LDM 15      ; restore carry
 ADD R7
 BBL 0       , return