Jump Instructions
The original EDSAC had two conditional jump instructions (and no
unconditional one!), namely Jump if Accumulator >= 0 and Jump if
Accumulator < 0. For implementing these, I added a new MISC value
called JUMP and two new UBCOND values for testing the SIGN output of
the ALU, called BSGN0 and BSGN1.
However, I didn't want to actually use these to perform microcode
branching, because then a jump instruction would require up to two
cycles, one to test the sign bit and one to load the PC if the test is
true. To save a cycle, I arranged things so that JUMP suppresses
microcode branching and instead activates a new signal LDPC during T17
if the microcode branch condition is true. This allows the jump to
occur at the end of the same cycle that tests the sign.
The microcode branch condition circuitry now looks like this:
LDPC causes the program counter to be loaded from the address field of the instruction register:
Here is the microcode for the jump instructions. Each one consists of
just a single microinstruction that passes the accumulator through the
ALU and activates JUMP together with the appropriate condition.
JUMP = 010 # Conditionally load PC from address field of instruction
BSGN0 = 110 # Sign FF = 0
BSGN1 = 001 # Sign FF = 1
# E - Jump if accumulator >= 0
00011 0 0001 : - EOI 11 XAC - Y0 CY0 -- - - MSW LSW - - - JUMP - BSGN0 ----
# G - Jump if accumulator < 0
11011 0 0001 : - EOI 11 XAC - Y0 CY0 -- - - MSW LSW - - - JUMP - BSGN1 ----
Update to Transfer and Clear instructions
While doing some more research into the EDSAC, I realised that my
implementations of the Transfer and Clear instructions weren't quite
right -- they were only clearing the parts of the accumulator that were
being written to memory, whereas they are supposed to clear the whole
accumulator. I've fixed them, but it means they now take 4 cycles to
execute. This is annoying, but there doesn't seem to be anything I can
easily do about it.
# T - Transfer and clear
00101 0 0001 : - - 00 XAC CMX YAC CY1 WAC - - - LSW - - - --- - --- ----
00101 0 0010 : - - 10 XAC CMX YAC -- WAC - - - - - - - --- - --- ----
00101 0 0011 : - - 01 XAC CMX YAC -- WAC - - - - - - - --- - --- ----
00101 0 0100 : - EOI 11 XAC CMX YAC -- WAC - - MSW - - - WMEM --- - --- ----
00101 1 0001 : - - 00 XAC CMX YAC CY1 WAC - - - LSW - - - --- - --- ----
00101 1 0010 : - - 10 XAC CMX YAC -- WAC - - - - - - - --- - --- ----
00101 1 0011 : - - 01 XAC CMX YAC -- WAC EVN - - - - - WMEM --- - --- ----
00101 1 0100 : - EOI 11 XAC CMX YAC -- WAC ODD - MSW - - - WMEM --- - --- ----