Intel 8080 - B4_Page_23

Chapter 6. Programming Techniques
Label
Code Operand
START:
LXI H,BTBL
;REGISTERS HAND L WILL
;POINT TO BRANCH TABLE
GTBIT:
RAR
IC
GETAD
INX
H
;(H,L)=(H,L)+2
TO
INX
H ;POINT TO NEXT ADDRESS
;IN BRANCH TABLE
JMP
GTBIT
GETAD: MOV
E,M ;BIT FOUND
INX
H ;LOAD I UMP ADDRESS
;INTO 0 AND E REGISTERS
MOV D,M
XCHG ;EXCHANGE 0 AND E
;WITH HAND L
PCHL
;1 UMP TO ROUTINE
;ADDRESS
BTBL:
OW ROUTl ;BRANCH TABLE. EACH
OW ROUT2 ;ENTRY IS A TWO-BYTE
OW ROUn
;ADDRESS
OW ROUT4
;HELD LEAST SIGNIFICANT
OW ROUTS ;BYTE FIRST
OW
ROUT6
OW ROUn
OW
ROUT8
The control routine at START uses the Hand L registers as a pointer into the branch table (BTBL) corresponding
to the bit of the accumulator that
is set. The routine at GET AO then transfers the address held in the corres-
ponding branch table entry to the Hand L registers via the 0 and E registers, and then uses a PCHL instruction,
thus transferring control to the selected routine.
TRANSFERRING DATA TO SUBROUTINES
A subroutine typically requires data to perform its operations. In the simplest case, this data may be transferred
in one or more registers.
Sometimes it
is more convenient and economical to let the subroutine load its own registers. One way to do this
is to place a list of the required data (called a parameter list) in some data area of memory, and pass the address
of this
list to the subroutine in the Hand L registers.
6-3