Z80 Programming


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Z80 Programming
For Mechatronics



Z80 5seconds Timer for 10 MHz
Point: Use 3 Loops
            ORG
0000H            
LD SP,0FFFFH  MAIN:   CALL TIMER5 TIMER5:LD E,35H
J60:      LD B,0FFH
J61:      LD D,0FFH
J62:      DEC D
            JP
NZ,J62            
DEC B            
JP NZ,J61
            DEC E
            JP
NZ,J60            
RET            
ENDnote: If your system has CTC, it's better to use CTC timer
interrupt. Z80 Mode 2 interrupt reduces CPU's load. 


Z80 Multi Vibrator
Point: Use Loop "output  00H and FFH at 1 second interval".
            ORG
0000H            
LD SP,0FFFFH
            PPI1
EQU 37H            
PORTB1 EQU 35H
            LD
A,90H            
OUT (PPI1),A LOOP:   LD A,0
            OUT
(PORTB1),A
            CALL
TIMER1            
LD A,0FFH
            OUT
(PORTB1),A
            CALL
TIMER1            
JP LOOP TIMER1:LD E,0AH J50:     LD B,0FFH
J51:     LD D,0FFH J52:    
DEC D            JP
NZ,J52            DEC B
           JP NZ,J51
           DEC E
           JP NZ,J50
           RET
          
END


Z80 Bad Coding
Z-80's ADD
When you write Z80 source code, remember... You can't write this.
ADC B,CBecause Z-80's ADC(or ADD) is used by A or HL register
only.
LD A,B ADC A,C LD B,AThis is OK.


Infinite Loop
Example code
            
............................
               
LD BC,0FFFFH LOOP:       DEC BC
               
JP NZ,LOOP
               
.............................
               
.............................  After this operation "DEC
BC(or INC BC)" does not set Z flag even BC is 0. So this example
code can't escape from infinite loop forever.  
note:BC, DE and HL are 16bits pair registers.

How to work "PUSH"
This example is very short program...
           
ORG     0000H
           
LD    SP,0FFFFH
           
LD    BC,0FFFFH LOOP:    PUSH BC
            JP LOOP
            END
 
STARTING MEMORY STATUS
0000    31 50 00    LD   
SP,FFFF 0003    01 FF FF   
LD    BC,FFFF 0006   
C5            PUSH BC
0007    C3 06 00   JP    LOOP
000A   
00           NOP
000B   
00           NOP
------------------- ------------------- -------------------
FFFF   
00           NOP  
MEMORY STATUS AFTER EXECUTION
0000    31 50 00    LD   
SP,FFFF 0003    01 FF FF   
LD    BC,FFFF 0006   
C5            PUSH BC
0007    C3 FF FF   JP    FFFF
; Changed! 000A   
FF           
RST    38 000B   
FF           
RST    38 ------------------------
------------------------ ------------------------
FFFF   
FF           
RST    38   note: "LD
BC,FFFF";Register BC is set FFFF. By "PUSH BC"top stack becomes
FFFF. Stack area moves to smaller memory address. After
all, the endless PUSH destroyed even program area.
This program is meaningless. But you will learn how to work
"PUSH" of Z-80's CPU.
                                                                        
(Sat,3/7/1998)

 Brunei's homework of Z-80 We got a brunei
student's homework of Z-80 by e-mail... Let's challenge this test!
Section A ( 40 marks)
Answer all questions.  
1. Comment on the Z80 instruction LD HL, SP. [1]
2. What is the effect of LD HL, 0 followed immediately by ADD HL, SP?
[2]
3. What does the instruction EX DE, HL do? [1]
4. What arithmetic �gtrick�h is associated with the instruction ADD A, A?
[2]
5. Is LD BC, HL a valid Z80 instruction? [1]
6. What instruction(s) would be needed to copy HL into BC? [2]
7. Comment on the instruction LD A, (DE) and LD B, (DE)? [2]
8. What happens when JP (HL) is executed? [3]
9. Briefly explain the actions taken by the Z80 on executing a CALL
instruction. [4]
10. What does RET instruction do?
11. How will the instruction OR L affect the Z80 flags? [3]
12. With the aid of a diagram, show how RCLA works. [2]  *
13. What condition is required for XOR B to make the sign flag 1 after it
is ran? [2]
14. If IX = 2000h what does the instruction LD (IX ? 7), A do? [3]
15. Briefly explain the operation of LDDR. [3]
16. Explain the effects of the NOP instruction. [1]
17. Comment on the instruction CPL A. [2]
18. Is SUB A, B a valid Z80 instruction? [1]
19. Explain the operation of LD A, (HL). [2]
20. What does LD IX, IY do? [1]
* RCLA is RLCA?

Hints 2. is the solution of 1. . 3. DE
<--> HL (Exchange with no flag change) 4. 2*A 5.,
6. and 20. No instructions in Z80. It is easy code to
use stack. 8. Jump to memory address HL shown. 10.Ret is
Return from subroutine. What's in the top stack? 12.Rotate left
circular accumulator 13.Check MSB of A and MSB of B  (1-0 or
0-1) 15. Instruction of memory block transfer    
Set register before this instruction LDDR.    
HL<-Start memory address     DE<-Destination address
    BC<-Data amount 16. Nop is often used as
debug of timing coordination. 17. Same instruction of XOR
18. SUB B or SBC A,B
06/01/1998


Z-80's Registers
Z-80 Primary registers



A(8bits); 
Accumulator
PSW(8bits);   Flag
register

B(8bits)
C(8bits)

D(8bits)
E(8bits)

H(8bits)
L(8bits)Z-80 Alternate registers



A'(8bits)
PSW'(8bits)

B'(8bits) 
C'(8bits)

D'(8bits)
E'(8bits)

H'(8bits)
L'(8bits) Z-80 Index registers




IX(16bits)


IY(16bits) Z-80 Other registers



I(8bits);Interrupt
register 
R(8bits);Refresh
register     Z-80 Flag
registers
    S(1bit) "sign flag" ;+ - of data with sign
    Z(1bit) "zero flag"
    H(1bit) "half carry";used at BCD calculation
    P(1bit) "parity flag" ;even or odd
    V (1bit)"overflow flag"
    N(1bit) "negative flag" add or dec
    C(1bit) "carry flag"


     Pseudo instructions
     ORG "origin"
     DEFB or DB "define
byte"
     DW "define word"
     EQU "equal to"
     END "end of program"



 IBM-PC Parallel Port Assignment




Pin #

Function


1
Strobe (Data Set Signal from PC)


2
Data bit 0


3
Data bit 1


4
Data bit 2


5
Data bit 3


6
Data bit 4


7
Data bit 5


8
Data bit 6


9
Data bit 7


10
Acknowledge(Printer to PC)


11
Busy


12
Paper end


13
Select


14
Auto feed


15
Error


16
Initialize-Input


17
Select-Input


18-25
Ground     
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