gc80c520a CORERIVER Semiconductor, gc80c520a Datasheet - Page 33

• Manufacturer Part Number: gc80c520a
• Description: Midas1.0 Family Mask Rom/eprom/romless 8-bit Turbo Microcontrollers
• Manufacturer: CORERIVER Semiconductor

It is not obvious that the last three instructions in this list are read-modify-write instructions, but they are.
They read the port byte, all 8 bits, modify the addressed bit, then write the new byte back to the latch.
The reason that read-modify-write instructions are directed to the latch rather than the pin is to avoid a
possible misinterpretation of the voltage level at the pin. For example, a port bit might be used to drive the
base of transistor. When logic 1 is written to the bit, the transistor is turned on. If the CPU the reads the
same port bit at the pin rather than the latch, it will read the base voltage of the transistor and interpret it
as logic 0. Reading the latch rather than the pin will return the correct value of logic 1.
6.2.1.10 Alternate Functions
All the port pins are multifunctional. They also serve the alternate functions specified below. The alternate
functions can only be activated if the corresponding bit latch in the port SFR contains logic 1. Otherwise
the port pin is stuck at 0. The alternate functions are activated respectively. They will be activated or
inactivated according to the application. More information about each alternate function will be provided
later.
MiDAS1.0 Family
Instruction
MOV PX.Y, C
SETB PX.Y
CLR PX.Y
DJNZ
ANL
ORL
DEC
XRL
JBC
CPL
INC
Table 6-4 Read-Modify-Write Instructions
Page 33 of 187
Description
Logical AND
Logical OR
Logical Exclusive OR (XOR)
Branch if bit is set then clear bit
Complement bit
Increment
Decrement
Decrement and branch if not zero
Move the carry bit to bit Y of Port X
Clear bit Y of Port X
Set bit Y of Port X