WPM
Name |
Write accumulator into RAM character |
Function |
Read/Write half a byte to Program RAM from accumulator. |
Syntax |
WPM |
Assembled |
|
Binary |
11100011 |
Decimal |
227 |
Hexadecimal |
0xE3 |
Symbolic |
|
Execution |
1 word, 8-bit code and an execution time of 10.3 \({\mu}\) sec |
Side-effects |
Not Applicable |
Implemented |
Detailed Description
This is a special instruction which may be used to write the contents of the accumulator into a half byte of program RAM,or read the contents of a half byte of program RAM into a ROM input port where it can be accessed by a program.
The carry bit is not affected.
The opcode for this instruction does not contain any additional data:
Notes
Two WPM instructions must always appear in close succession; that is, each time one WPM instruction references a half byte of program RAM as indicated by an SRC address, another WPM must access the other half byte before the SRC address is altered. An internal counter keeps track of which half-byte is being accessed. If only one WPM occurs, this ounter will be out of sync with the program and errors will occur. In this situation a RESET pulse must be used to re-initialize the machine.
A WPM instruction requires an SRC address to access program RAM. Whenever a WPM is executed, the DATA RAM which happens to correspond to this SRC address will also be written. If data needed later in the program is being held in such DATA RAM, the programmer must save it elsewhere before executing the WPM instruction.
Storing Data Into Program RAM
A program must perform the following actions in order to store eight bits of data into a program RAM location:
The value 1 must be written to ROM port number 14. This is a “write enable” signal, permitting the store operation to work.
The highest 4 bits of the program RAM address to be accessed must be written to ROM port number 15.
The lowest 8 bits of the program RAM address to be accessed must be sent out by an SRC instruction.
The higher 4 bits of data to be written must be loaded into the accumulator and written with the first WPM; the lower 4 bits of data must then be loaded into the accumulator and written with the second WPM.
The value 0 must be written to ROM port number 14, clearing the “write enable”.
Reading Data From Program RAM
A program must perform the following actions in order to read eight bits of data from a program RAM location:
The highest 4 bits of the program RAM address to be accessed must be written to ROM port 15.
The lowest 8 bits of the program RAM address to be accessed must be sent out by an SRC instruction.
Two WPM instructions in succession must be executed. The first reads the leftmost 4 bits of the program RAM location into ROM port 14; the second reads the rightmost 4 bits of the program RAM location into ROM port 15.
Example Program
The following program writes to a program RAM location whose address is held in status characters 0, 1, and 2 of DATA RAM register 0 of DATA RAM chip 0, shown below.
/ Example program
FIM 0P 180
SRC 0P
LDM 15
WRM
FIM 0P 224
SRC 0P / Select ROM port 14.
LDM 1
WRR / Turn on write enable.
/ Set up PRAM address.
/
FIM 0P 0
SRC 0P / Select DATA RAM chip 0 register 0.
RD1 / Read middle 4 bits of address.
XCH 10 / Save in register 10.
RD2 / Read lowest 4 bits of address.
XCH 11 / Save in register 11.
RD0 / Read highest 4 bits of address.
FIM 0P 240
SRC 0P / Select ROM port 15.
WRR / Write high address.
SRC 5P / Write middle + low address (RP5)
/
LD 2 / High 4 data bits to accumulator.
WPM / Write to PRAM
LD 3 / Low 4 data bits to accumulator.
WPM / Write to PRAM
FIM 0P 224
SRC 0P / Select ROM port 14.
CLB
WRR / Turn off write enable.
