Zilog Z08470 - Manual (Page 292)

Z80 Instruction Set UM008007-0715
280
Z80 CPU
User Manual
CALL cc, nn
Operation
IF cc true: (sp – 1) ← PCH
(sp – 2) ← PCL, pc ← nn
Op Code
CALL
Operands
cc, nn
The first of the two n operands in the assembled object code above is the least-significant
byte of the 2-byte memory address.
Description
If condition cc is true, this instruction pushes the current contents of the Program Counter
(PC) onto the top of the external memory stack, then loads the operands nn to PC to point
to the address in memory at which the first op code of a subroutine is to be fetched. At the
end of the subroutine, a RETurn instruction can be used to return to the original program
flow by popping the top of the stack back to PC. If condition cc is false, the Program
Counter is incremented as usual, and the program continues with the next sequential
instruction. The stack push is accomplished by first decrementing the current contents of
the Stack Pointer (SP), loading the high-order byte of the PC contents to the memory
address now pointed to by SP; then decrementing SP again, and loading the low-order
byte of the PC contents to the top of the stack.
Because this process is a 3-byte instruction, the Program Counter was incremented by
three before the push is executed.
Condition cc is programmed as one of eight statuses that corresponds to condition bits in
the Flag Register (Register F). These eight statuses are defined in the following table,
which also specifies the corresponding cc bit fields in the assembled object code.
n
n
1 1 0 1 0 cc