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Added BRS and RTN instructions.

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Russell-S-Harper
2018-09-17 20:35:19 -04:00
parent 7ac0d7e04a
commit f7ee11763b
2 changed files with 71 additions and 32 deletions
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90
common/common.asm
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@@ -371,7 +371,7 @@ _TRFI0X .( ; transfer I0 to register indexed by X, MSB returned in A
RTS
.)
_RTZI0X .( ; clears I0, clears underflow, falls through to _RETI0X
_RTZI0X .( ; clears I0, clears underflow, *falls thru* to _RETI0X
JSR _ZERI0
LDA _F ; clear underflow
AND #_F_U^$FF
@@ -841,40 +841,35 @@ _ESC .( ; ESC 00 - escape back into regular assembler
.)
_RTN .( ; RTN 01 - return from subroutine
; some stack hocus pocus
; currently the stack has return address corresponding to _CMN._1 + 2, _PCL, _PCH, ...
; we need to pop that, pop the updated PC, then push the address back
PLA ; save the return address
STA _I0+2
PLA
STA _I0+3
PLA ; pull the program counter to return
STA _PCH
PLA
STA _PCL
LDA _I0+3 ; push the return address back
PHA
LDA _I0+2
PHA
RTS
.)
_BRS .( ; BRS xxyy 02 yy xx PC <- PC + xxyy - branch to subroutine
RTS
.)
_BRA .( ; BRA xxyy 03 yy xx PC <- PC + xxyy - branch always
_INIBR .( ; save the branch offset in I0, *falls thru* to _UPDPC
LDY #1 ; save the offset
LDA (_PC), Y
PHA
STA _I0+1 ; high byte
DEY
LDA (_PC), Y
PHA
LDA #2 ; update PC by the length of the branch address
CLC
ADC _PCL
STA _PCL
BNE _1
INC _PCH
_1 PLA ; pull the low byte
CLC
ADC _PCL
STA _PCL
PLA ; pull the high byte
ADC _PCH
STA _PCH
RTS
STA _I0 ; low byte
.)
_BRX .( ; generic branch testing
AND _F ; check the bit
BNE _BRA ; if set, branch
LDA #2 ; not set, advance the program counter over the xxyy offset
_UPDPC .( ; update PC by the length of a branch address
LDA #2 ; the length of a branch address
CLC
ADC _PCL
STA _PCL
@@ -883,6 +878,49 @@ _BRX .( ; generic branch testing
_1 RTS
.)
_UPPCI0 .( ; update PC with offset in I0
LDA _I0 ; get the low byte
CLC
ADC _PCL
STA _PCL
LDA _I0+1 ; get the high byte
ADC _PCH
STA _PCH
RTS
.)
_BRS .( ; BRS xxyy 02 yy xx PC <- PC + xxyy - branch to subroutine
; some stack hocus pocus
; currently the stack has return address corresponding to _CMN._1 + 2, ...
; we need to pop that, push the updated PC, then push the address back
JSR _INIBR ; save the offset and update PC by the length of a branch address
PLA
STA _I0+2
PLA
STA _I0+3
LDA _PCL ; push the program counter to return
PHA
LDA _PCH
PHA
LDA _I0+3 ; push the return address back
PHA
LDA _I0+2
PHA
; stack is now _CMN._1 + 2, _PCL, _PCH, ...
JMP _UPPCI0 ; update PC with I0 offset, let it handle the return
.)
_BRA .( ; BRA xxyy 03 yy xx PC <- PC + xxyy - branch always
JSR _INIBR ; save the offset and update PC by the length of a branch address
JMP _UPPCI0 ; update PC with I0 offset, let it handle the return
.)
_BRX .( ; generic branch testing
AND _F ; check the bit
BNE _BRA ; if set, branch
JMP _UPDPC ; not set, advance the program counter over the xxyy offset, let it handle the return
.)
_BRE .( ; BRE xxyy 04 yy xx PC <- PC + xxyy - branch if Rp = Rq (after CMR)
LDA #_F_E
BNE _BRX